diff --git a/CMakeLists.txt b/CMakeLists.txt
index b02d7e458..ecb9a12c3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -79,12 +79,12 @@ endif()
 add_library(${PROJECT_NAME} OBJECT ${C2A_SRCS})
 
 if(USE_ALL_C2A_CORE_APPS)
-  add_subdirectory(Applications)
+  add_subdirectory(applications)
   target_sources(${PROJECT_NAME} PUBLIC $<TARGET_OBJECTS:C2A_CORE_APPS>)
 endif()
 
 if(USE_ALL_C2A_CORE_TEST_APPS)
-  add_subdirectory(Applications/TestApp)
+  add_subdirectory(applications/test_app)
   target_sources(${PROJECT_NAME} PUBLIC $<TARGET_OBJECTS:C2A_CORE_TEST_APPS>)
 endif()
 
diff --git a/System/ApplicationManager/app_manager.c b/System/ApplicationManager/app_manager.c
index 34d93cb19..6ee077794 100644
--- a/System/ApplicationManager/app_manager.c
+++ b/System/ApplicationManager/app_manager.c
@@ -8,7 +8,7 @@
 #include <src_user/Settings/sils_define.h>
 
 #include <string.h>
-#include "../../Applications/nop.h"
+#include "../../applications/nop.h"
 #include "../EventManager/event_logger.h"
 #include "../TimeManager/time_manager.h"
 #include "../WatchdogTimer/watchdog_timer.h"
diff --git a/System/EventManager/event_handler.c b/System/EventManager/event_handler.c
index afde3f4d6..d22f10912 100644
--- a/System/EventManager/event_handler.c
+++ b/System/EventManager/event_handler.c
@@ -8,7 +8,7 @@
 #include <string.h>
 #include <stdlib.h>
 #include "../../TlmCmd/common_cmd_packet_util.h"
-#include "../../Applications/timeline_command_dispatcher_id_define.h"
+#include "../../applications/timeline_command_dispatcher_id_define.h"
 #include "../TimeManager/time_manager.h"
 
 #ifdef EL_IS_ENABLE_TLOG
diff --git a/System/ModeManager/mode_manager.c b/System/ModeManager/mode_manager.c
index 1a7f1111c..08764edd5 100644
--- a/System/ModeManager/mode_manager.c
+++ b/System/ModeManager/mode_manager.c
@@ -14,7 +14,7 @@
 #include "../../TlmCmd/common_cmd_packet_util.h"
 #include <src_user/TlmCmd/command_definitions.h>
 #include "../../TlmCmd/packet_handler.h"
-#include "../../Applications/timeline_command_dispatcher_id_define.h"
+#include "../../applications/timeline_command_dispatcher_id_define.h"
 #include "../../Library/endian.h"
 
 /**
diff --git a/TlmCmd/block_command_loader.c b/TlmCmd/block_command_loader.c
index c5d6d0331..3e74e33ea 100644
--- a/TlmCmd/block_command_loader.c
+++ b/TlmCmd/block_command_loader.c
@@ -9,7 +9,7 @@
 
 #include <string.h>
 #include "block_command_loader.h"
-#include "../Applications/timeline_command_dispatcher_id_define.h"
+#include "../applications/timeline_command_dispatcher_id_define.h"
 #include "block_command_executor.h"
 #include <src_user/TlmCmd/command_definitions.h> // for rotate/combine block
 #include "common_cmd_packet_util.h"
diff --git a/TlmCmd/block_command_loader.h b/TlmCmd/block_command_loader.h
index d0a34828d..3d53d939a 100644
--- a/TlmCmd/block_command_loader.h
+++ b/TlmCmd/block_command_loader.h
@@ -9,9 +9,9 @@
 #define BLOCK_COMMAND_LOADER_H_
 
 #include "./block_command_table.h"
-#include "../Applications/timeline_command_dispatcher_id_define.h"
+#include "../applications/timeline_command_dispatcher_id_define.h"
 #include "../System/TimeManager/obc_time.h"
-#include <src_user/Applications/app_registry.h>
+#include <src_user/applications/app_registry.h>
 
 
 /**
diff --git a/TlmCmd/common_cmd_packet.h b/TlmCmd/common_cmd_packet.h
index cadde6930..152197d8e 100644
--- a/TlmCmd/common_cmd_packet.h
+++ b/TlmCmd/common_cmd_packet.h
@@ -33,7 +33,7 @@ typedef enum
 #include <src_user/Settings/TlmCmd/Ccsds/apid_define.h>
 
 // TL_MIS を有効にするかどうか
-#include <src_user/Settings/Applications/timeline_command_dispatcher_define.h>
+#include <src_user/Settings/applications/timeline_command_dispatcher_define.h>
 
 /**
  * @enum   CCP_EXEC_STS
diff --git a/TlmCmd/common_cmd_packet_util.h b/TlmCmd/common_cmd_packet_util.h
index 4775d3b16..d0d89b763 100644
--- a/TlmCmd/common_cmd_packet_util.h
+++ b/TlmCmd/common_cmd_packet_util.h
@@ -7,9 +7,9 @@
 
 #include "common_cmd_packet.h"
 #include "packet_handler.h"
-#include "../Applications/timeline_command_dispatcher_id_define.h"
+#include "../applications/timeline_command_dispatcher_id_define.h"
 #include "block_command_table.h" // for bct_id
-#include <src_user/Applications/app_registry.h>
+#include <src_user/applications/app_registry.h>
 
 /**
  * @enum   CCP_UTIL_ACK
diff --git a/TlmCmd/packet_handler.h b/TlmCmd/packet_handler.h
index 6ef225cc6..ea99c6128 100644
--- a/TlmCmd/packet_handler.h
+++ b/TlmCmd/packet_handler.h
@@ -7,7 +7,7 @@
 
 #include "common_tlm_cmd_packet.h"
 #include "packet_list.h"
-#include "../Applications/timeline_command_dispatcher_id_define.h"
+#include "../applications/timeline_command_dispatcher_id_define.h"
 
 #define TL_TLM_PAGE_SIZE  (32)
 #define TL_TLM_PAGE_MAX   (8)
diff --git a/Applications/CMakeLists.txt b/applications/CMakeLists.txt
similarity index 100%
rename from Applications/CMakeLists.txt
rename to applications/CMakeLists.txt
diff --git a/Applications/divided_cmd_utility.c b/applications/divided_cmd_utility.c
similarity index 100%
rename from Applications/divided_cmd_utility.c
rename to applications/divided_cmd_utility.c
diff --git a/Applications/divided_cmd_utility.h b/applications/divided_cmd_utility.h
similarity index 99%
rename from Applications/divided_cmd_utility.h
rename to applications/divided_cmd_utility.h
index 253a5c7bd..e314fb405 100644
--- a/Applications/divided_cmd_utility.h
+++ b/applications/divided_cmd_utility.h
@@ -21,7 +21,7 @@
 #define DCU_LOG_MAX    (16)    //!< 保存するログの最大数
 
 // DCU_LOG_MAX の user オーバーライド
-#include <src_user/Settings/Applications/divided_cmd_utility_params.h>
+#include <src_user/Settings/applications/divided_cmd_utility_params.h>
 
 // key が uint8_t なので，それ以下を要請する
 #if DCU_LOG_MAX > 255
diff --git a/Applications/event_utility.c b/applications/event_utility.c
similarity index 100%
rename from Applications/event_utility.c
rename to applications/event_utility.c
diff --git a/Applications/event_utility.h b/applications/event_utility.h
similarity index 100%
rename from Applications/event_utility.h
rename to applications/event_utility.h
diff --git a/Applications/gs_command_dispatcher.c b/applications/gs_command_dispatcher.c
similarity index 100%
rename from Applications/gs_command_dispatcher.c
rename to applications/gs_command_dispatcher.c
diff --git a/Applications/gs_command_dispatcher.h b/applications/gs_command_dispatcher.h
similarity index 100%
rename from Applications/gs_command_dispatcher.h
rename to applications/gs_command_dispatcher.h
diff --git a/Applications/memory_dump.c b/applications/memory_dump.c
similarity index 100%
rename from Applications/memory_dump.c
rename to applications/memory_dump.c
diff --git a/Applications/memory_dump.h b/applications/memory_dump.h
similarity index 100%
rename from Applications/memory_dump.h
rename to applications/memory_dump.h
diff --git a/Applications/nop.c b/applications/nop.c
similarity index 100%
rename from Applications/nop.c
rename to applications/nop.c
diff --git a/Applications/nop.h b/applications/nop.h
similarity index 100%
rename from Applications/nop.h
rename to applications/nop.h
diff --git a/Applications/realtime_command_dispatcher.c b/applications/realtime_command_dispatcher.c
similarity index 100%
rename from Applications/realtime_command_dispatcher.c
rename to applications/realtime_command_dispatcher.c
diff --git a/Applications/realtime_command_dispatcher.h b/applications/realtime_command_dispatcher.h
similarity index 100%
rename from Applications/realtime_command_dispatcher.h
rename to applications/realtime_command_dispatcher.h
diff --git a/Applications/telemetry_manager.c b/applications/telemetry_manager.c
similarity index 99%
rename from Applications/telemetry_manager.c
rename to applications/telemetry_manager.c
index 3ff65139a..a35683df5 100644
--- a/Applications/telemetry_manager.c
+++ b/applications/telemetry_manager.c
@@ -34,8 +34,8 @@
 #define TLM_MGR_BC_ROLE_AT_BC_9   (TLM_MGR_BC_ROLE_LOW_FREQ_TLM)    // TLM_MGR_BC_ROLE_HIGH_FREQ_TLM が固まらないようにここに入れている
 
 // user 設定
-#include <src_user/Settings/Applications/telemetry_manager_define.h>
-#include <src_user/Settings/Applications/telemetry_manager_params.h>
+#include <src_user/Settings/applications/telemetry_manager_define.h>
+#include <src_user/Settings/applications/telemetry_manager_params.h>
 
 // 現状， BC#9 は low ではないとだめ (deploy と combine するため)
 #if !(TLM_MGR_BC_ROLE_AT_BC_9 == TLM_MGR_BC_ROLE_LOW_FREQ_TLM)
diff --git a/Applications/telemetry_manager.h b/applications/telemetry_manager.h
similarity index 100%
rename from Applications/telemetry_manager.h
rename to applications/telemetry_manager.h
diff --git a/Applications/TestApp/CMakeLists.txt b/applications/test_app/CMakeLists.txt
similarity index 100%
rename from Applications/TestApp/CMakeLists.txt
rename to applications/test_app/CMakeLists.txt
diff --git a/Applications/TestApp/test_ccp_util.c b/applications/test_app/test_ccp_util.c
similarity index 100%
rename from Applications/TestApp/test_ccp_util.c
rename to applications/test_app/test_ccp_util.c
diff --git a/Applications/TestApp/test_ccp_util.h b/applications/test_app/test_ccp_util.h
similarity index 100%
rename from Applications/TestApp/test_ccp_util.h
rename to applications/test_app/test_ccp_util.h
diff --git a/Applications/timeline_command_dispatcher.c b/applications/timeline_command_dispatcher.c
similarity index 100%
rename from Applications/timeline_command_dispatcher.c
rename to applications/timeline_command_dispatcher.c
diff --git a/Applications/timeline_command_dispatcher.h b/applications/timeline_command_dispatcher.h
similarity index 100%
rename from Applications/timeline_command_dispatcher.h
rename to applications/timeline_command_dispatcher.h
diff --git a/Applications/timeline_command_dispatcher_id_define.h b/applications/timeline_command_dispatcher_id_define.h
similarity index 90%
rename from Applications/timeline_command_dispatcher_id_define.h
rename to applications/timeline_command_dispatcher_id_define.h
index 53bbcb9a4..355b74475 100644
--- a/Applications/timeline_command_dispatcher_id_define.h
+++ b/applications/timeline_command_dispatcher_id_define.h
@@ -6,7 +6,7 @@
 #ifndef TIMELINE_COMMAND_DISPATCHER_ID_DEFINE_H_
 #define TIMELINE_COMMAND_DISPATCHER_ID_DEFINE_H_
 
-#include <src_user/Settings/Applications/timeline_command_dispatcher_define.h>
+#include <src_user/Settings/applications/timeline_command_dispatcher_define.h>
 
 /**
  * @enum   TLCD_ID
diff --git a/Applications/utility_command.c b/applications/utility_command.c
similarity index 100%
rename from Applications/utility_command.c
rename to applications/utility_command.c
diff --git a/Applications/utility_command.h b/applications/utility_command.h
similarity index 100%
rename from Applications/utility_command.h
rename to applications/utility_command.h
diff --git a/Applications/utility_counter.c b/applications/utility_counter.c
similarity index 100%
rename from Applications/utility_counter.c
rename to applications/utility_counter.c
diff --git a/Applications/utility_counter.h b/applications/utility_counter.h
similarity index 100%
rename from Applications/utility_counter.h
rename to applications/utility_counter.h
diff --git a/c2a_core_main.c b/c2a_core_main.c
index 1f76a9d12..42b0a1b33 100644
--- a/c2a_core_main.c
+++ b/c2a_core_main.c
@@ -14,7 +14,7 @@
 #include "./TlmCmd/command_analyze.h"
 #include "./TlmCmd/telemetry_frame.h"
 
-#include <src_user/Applications/app_registry.h>
+#include <src_user/applications/app_registry.h>
 
 // git revisionをコードに埋め込む
 const char GIT_REV_CORE[41]         = GIT_REVISION_C2A_CORE;
diff --git a/docs/general/coding_rule.md b/docs/general/coding_rule.md
index 929b26cad..176021dc9 100644
--- a/docs/general/coding_rule.md
+++ b/docs/general/coding_rule.md
@@ -213,14 +213,14 @@ typedef uint32_t flash_block_t;
 
 
 ## 個別箇所についての命名など [M]
-### Applications/UserDefined
+### applications/user_defined
 - ファイル名と AppInfo 構造体名を一致させる（スタイルを除く）
 - AppInfo 構造体名とそのインスタンス名を一致させる（スタイルを除く）
 - 接頭辞は， `APP_HOGE` で `HOGE` 部分はケースバイケース．
 
 例：
 
-Applications/UserDefined/data_recorder.c
+applications/user_defined/data_recorder.c
 ```cpp
 static DataRecorder data_recorder_;
 const DataRecorder* const data_recorder = &data_recorder_;
@@ -232,7 +232,7 @@ AppInfo APP_DR_create_app(void)
 }
 ```
 
-Applications/UserDefined/data_recorder.h
+applications/user_defined/data_recorder.h
 ```cpp
 #ifndef DATA_RECORDER_H_
 #define DATA_RECORDER_H_
@@ -253,7 +253,7 @@ int Cmd_APP_DR_SET_PARAMS(const CommonCmdPacket* packet);
 ```
 
 
-### Applications/DriverInstances
+### applications/driver_instances
 - ファイル名は `di_${IFやデバイス名}`
 - Driver 構造体名とそのインスタンス名を一致させる（スタイルを除く）
     - 特定のドライバ構造体のインスタンスが複数ある場合は，配列にまとめる．
@@ -292,7 +292,7 @@ RM3100_Driver rm3100_driver[RM3100_IDX_MAX];
 ```
 
 
-### Applications/Middleware
+### applications/middleware
 - ファイル名は `mw_${IFやデバイス名}`
 - 接頭辞は `MW_${IFやデバイス名}`
 
@@ -312,7 +312,7 @@ const MwFlash* const mw_flash = &mw_flash_;
 ```
 
 
-### Applications その他
+### applications その他
 `AR_APP_ID` enumの接頭辞は，
 - `AR_NOP`: NOP
 - `AR_APP_HOGE`: UserDefined
@@ -322,8 +322,8 @@ const MwFlash* const mw_flash = &mw_flash_;
 とし，基本的には `HOGE` は ファイル名にする．
 
 つまり， 
-- Applications/UserDefined/data_recorder.h → `AR_APP_DATA_RECORDER`
-- Applications/DriverInstances/di_pcdu.h → `AR_DI_PCDU`
+- applications/user_defined/data_recorder.h → `AR_APP_DATA_RECORDER`
+- applications/driver_instances/di_pcdu.h → `AR_DI_PCDU`
 
 となる．
 
diff --git a/script/add_section/settings.pl b/script/add_section/settings.pl
index 5684fabb2..333dfa68a 100644
--- a/script/add_section/settings.pl
+++ b/script/add_section/settings.pl
@@ -16,7 +16,7 @@ sub GetSetting {
 	$SETTING{'FILE_ENCODING'} = 'utf8';
 	$SETTING{'LOG_FILE'}  = './log.log';
 	$SETTING{'SEARCH_PATH'} = [
-		'Applications',
+		'applications',
 		'Drivers',
 		'hal',
 		'Library',
diff --git a/script/ci/check_encoding.json b/script/ci/check_encoding.json
index 831faad77..8b4dd6a43 100644
--- a/script/ci/check_encoding.json
+++ b/script/ci/check_encoding.json
@@ -31,7 +31,7 @@
     ]
   },
   "target_dirs" : [
-    "Applications/",
+    "applications/",
     "Drivers/",
     "hal/",
     "Library/",
diff --git a/script/migration/v4-rename-applications.sh b/script/migration/v4-rename-applications.sh
new file mode 100755
index 000000000..be57b23d5
--- /dev/null
+++ b/script/migration/v4-rename-applications.sh
@@ -0,0 +1,49 @@
+#!/bin/bash
+# Script -> script
+
+## c2a-core ref
+find . -name "*" -not -path "*/.git/*" -type f -print0 | xargs -0 sed -i -e "s#src_core/Applications/TestApp#src_core/applications/test_app#g"
+find . -name "*" -not -path "*/.git/*" -type f -print0 | xargs -0 sed -i -e "s#src_core\\\Applications\\\TestApp#src_core\\\applications\\\test_app#g"
+
+find . -name "*" -not -path "*/.git/*" -type f -print0 | xargs -0 sed -i -e "s#src_core/Applications#src_core/applications#g"
+find . -name "*" -not -path "*/.git/*" -type f -print0 | xargs -0 sed -i -e "s#src_core\\\Applications#src_core\\\applications#g"
+
+find . -name "*" -not -path "*/.git/*" -type f -print0 | xargs -0 sed -i -e "s#\${C2A_CORE_DIR}/Applications#\${C2A_CORE_DIR}/applications#g"
+
+# C2A user code
+mv src/src_user/Applications src/src_user/applications
+
+mv src/src_user/applications/DriverInstances src/src_user/applications/driver_instances
+mv src/src_user/applications/Middleware src/src_user/applications/middleware
+mv src/src_user/applications/UserDefined src/src_user/applications/user_defined
+
+find . -name "*" -not -path "*/.git/*" -type f -print0 | xargs -0 sed -i -e "s#Applications/DriverInstances#applications/driver_instances#g"
+find . -name "*" -not -path "*/.git/*" -type f -print0 | xargs -0 sed -i -e "s#Applications\\\DriverInstances#applications\\\driver_instances#g"
+
+find . -name "*" -not -path "*/.git/*" -type f -print0 | xargs -0 sed -i -e "s#Applications/Middleware#applications/middleware#g"
+find . -name "*" -not -path "*/.git/*" -type f -print0 | xargs -0 sed -i -e "s#Applications\\\Middleware#applications\\\middleware#g"
+
+find . -name "*" -not -path "*/.git/*" -type f -print0 | xargs -0 sed -i -e "s#Applications/UserDefined#applications/user_defined#g"
+find . -name "*" -not -path "*/.git/*" -type f -print0 | xargs -0 sed -i -e "s#Applications\\\UserDefined#applications\\\user_defined#g"
+
+find . -name "*" -not -path "*/.git/*" -type f -print0 | xargs -0 sed -i -e "s#/Applications#/applications#g"
+find . -name "*" -not -path "*/.git/*" -type f -print0 | xargs -0 sed -i -e "s#\\\Applications#\\\applications#g"
+
+# CMake
+find . -name "*" -not -path "*/.git/*" -type f -print0 | xargs -0 sed -i -e "s#DriverInstances/#driver_instances/#g"
+find . -name "*" -not -path "*/.git/*" -type f -print0 | xargs -0 sed -i -e "s#Middleware/#middleware/#g"
+find . -name "*" -not -path "*/.git/*" -type f -print0 | xargs -0 sed -i -e "s#UserDefined/#user_defined/#g"
+
+# Settings
+mv src/src_user/Settings/Applications src/src_user/Settings/applications
+
+# Test
+mv src/src_user/Test/test/src_core/Applications src/src_user/Test/test/src_core/applications
+
+mv src/src_user/Test/test/src_user/Applications src/src_user/Test/test/src_user/applications
+mv src/src_user/Test/test/src_user/Applications/DriverInstances src/src_user/Test/test/src_user/applications/driver_instances
+mv src/src_user/Test/test/src_user/Applications/Middleware src/src_user/Test/test/src_user/applications/middleware
+mv src/src_user/Test/test/src_user/Applications/UserDefined src/src_user/Test/test/src_user/applications/user_defined
+
+# Script
+find src/src_user/script -name "*" -not -path "*/.git/*" -type f -print0 | xargs -0 sed -i -e "s#Applications#applications#g"