diff --git a/lib/fluent/log.rb b/lib/fluent/log.rb
index d7396ee4be..0e8c647c46 100644
--- a/lib/fluent/log.rb
+++ b/lib/fluent/log.rb
@@ -565,7 +565,7 @@ def event(level, args)
       }
 
       map.each_pair {|k,v|
-        if k == "error".freeze && v.is_a?(Exception) && !map.has_key?("error_class")
+        if k == "error" && v.is_a?(Exception) && !map.has_key?("error_class")
           message << " error_class=#{v.class.to_s} error=#{v.to_s.inspect}"
         else
           message << " #{k}=#{v.inspect}"
@@ -600,7 +600,7 @@ def caller_line(type, time, depth, level)
       worker_id_part = if type == :default && (@process_type == :worker0 || @process_type == :workers)
                          @worker_id_part
                        else
-                         "".freeze
+                         ""
                        end
       log_msg = "#{format_time(time)} [#{LEVEL_TEXT[level]}]: #{worker_id_part}"
       if @debug_mode
diff --git a/lib/fluent/plugin/formatter_csv.rb b/lib/fluent/plugin/formatter_csv.rb
index 6bc33db816..99c823bd38 100644
--- a/lib/fluent/plugin/formatter_csv.rb
+++ b/lib/fluent/plugin/formatter_csv.rb
@@ -29,7 +29,7 @@ class CsvFormatter < Formatter
       helpers :record_accessor
 
       config_param :delimiter, default: ',' do |val|
-        ['\t', 'TAB'].include?(val) ? "\t".freeze : val.freeze
+        ['\t', 'TAB'].include?(val) ? "\t" : val.freeze
       end
       config_param :force_quotes, :bool, default: true
       # "array" looks good for type of :fields, but this implementation removes tailing comma
diff --git a/lib/fluent/plugin/formatter_ltsv.rb b/lib/fluent/plugin/formatter_ltsv.rb
index 1bf054ecdf..3d5a3ab60e 100644
--- a/lib/fluent/plugin/formatter_ltsv.rb
+++ b/lib/fluent/plugin/formatter_ltsv.rb
@@ -27,9 +27,9 @@ class LabeledTSVFormatter < Formatter
 
       # http://ltsv.org/
 
-      config_param :delimiter, :string, default: "\t".freeze
-      config_param :label_delimiter, :string, default: ":".freeze
-      config_param :replacement, :string, default: " ".freeze
+      config_param :delimiter, :string, default: "\t"
+      config_param :label_delimiter, :string, default: ":"
+      config_param :replacement, :string, default: " "
       config_param :add_newline, :bool, default: true
 
       def format(tag, time, record)
diff --git a/lib/fluent/plugin/formatter_out_file.rb b/lib/fluent/plugin/formatter_out_file.rb
index bb853d5150..aca91e0b02 100644
--- a/lib/fluent/plugin/formatter_out_file.rb
+++ b/lib/fluent/plugin/formatter_out_file.rb
@@ -31,9 +31,9 @@ class OutFileFormatter < Formatter
       config_param :output_tag, :bool, default: true
       config_param :delimiter, default: "\t" do |val|
         case val
-        when /SPACE/i then ' '.freeze
-        when /COMMA/i then ','.freeze
-        else "\t".freeze
+        when /SPACE/i then ' '
+        when /COMMA/i then ','
+        else "\t"
         end
       end
       config_set_default :time_type, :string
diff --git a/lib/fluent/plugin/in_http.rb b/lib/fluent/plugin/in_http.rb
index 1f72f32b4d..59501cea51 100644
--- a/lib/fluent/plugin/in_http.rb
+++ b/lib/fluent/plugin/in_http.rb
@@ -195,11 +195,11 @@ def close
     end
 
     RES_TEXT_HEADER = {'Content-Type' => 'text/plain'}.freeze
-    RESPONSE_200 = ["200 OK".freeze, RES_TEXT_HEADER, "".freeze].freeze
-    RESPONSE_204 = ["204 No Content".freeze, {}.freeze].freeze
-    RESPONSE_IMG = ["200 OK".freeze, {'Content-Type'=>'image/gif; charset=utf-8'}.freeze, EMPTY_GIF_IMAGE].freeze
-    RES_400_STATUS = "400 Bad Request".freeze
-    RES_500_STATUS = "500 Internal Server Error".freeze
+    RESPONSE_200 = ["200 OK", RES_TEXT_HEADER, ""].freeze
+    RESPONSE_204 = ["204 No Content", {}.freeze].freeze
+    RESPONSE_IMG = ["200 OK", {'Content-Type'=>'image/gif; charset=utf-8'}.freeze, EMPTY_GIF_IMAGE].freeze
+    RES_400_STATUS = "400 Bad Request"
+    RES_500_STATUS = "500 Internal Server Error"
 
     def on_request(path_info, params)
       begin
@@ -306,7 +306,7 @@ def parse_params_with_parser(params)
     def add_params_to_record(record, params)
       if @add_http_headers
         params.each_pair { |k, v|
-          if k.start_with?("HTTP_".freeze)
+          if k.start_with?("HTTP_")
             record[k] = v
           end
         }
@@ -314,7 +314,7 @@ def add_params_to_record(record, params)
 
       if @add_query_params
         params.each_pair { |k, v|
-          if k.start_with?("QUERY_".freeze)
+          if k.start_with?("QUERY_")
             record[k] = v
           end
         }
@@ -422,7 +422,7 @@ def on_headers_complete(headers)
           end
         }
         if expect
-          if expect == '100-continue'.freeze
+          if expect == '100-continue'
             if !size || size < @body_size_limit
               send_response_nobody("100 Continue", {})
             else
@@ -444,8 +444,8 @@ def on_body(chunk)
         @body << chunk
       end
 
-      RES_200_STATUS = "200 OK".freeze
-      RES_403_STATUS = "403 Forbidden".freeze
+      RES_200_STATUS = "200 OK"
+      RES_403_STATUS = "403 Forbidden"
 
       # Azure App Service sends GET requests for health checking purpose.
       # Respond with `200 OK` to accommodate it.
@@ -489,11 +489,11 @@ def handle_options_request
       def on_message_complete
         return if closing?
 
-        if @parser.http_method == 'GET'.freeze
+        if @parser.http_method == 'GET'
           return handle_get_request()
         end
 
-        if @parser.http_method == 'OPTIONS'.freeze
+        if @parser.http_method == 'OPTIONS'
           return handle_options_request()
         end
 
@@ -513,9 +513,9 @@ def on_message_complete
         # Decode payload according to the "Content-Encoding" header.
         # For now, we only support 'gzip' and 'deflate'.
         begin
-          if @content_encoding == 'gzip'.freeze
+          if @content_encoding == 'gzip'
             @body = Zlib::GzipReader.new(StringIO.new(@body)).read
-          elsif @content_encoding == 'deflate'.freeze
+          elsif @content_encoding == 'deflate'
             @body = Zlib::Inflate.inflate(@body)
           end
         rescue
@@ -576,7 +576,7 @@ def on_message_complete
         end
 
         if @keep_alive
-          header['Connection'] = 'Keep-Alive'.freeze
+          header['Connection'] = 'Keep-Alive'
           send_response(code, header, body)
         else
           send_response_and_close(code, header, body)
@@ -602,13 +602,13 @@ def closing?
 
       def send_response(code, header, body)
         header['Content-Length'] ||= body.bytesize
-        header['Content-Type'] ||= 'text/plain'.freeze
+        header['Content-Type'] ||= 'text/plain'
 
         data = +"HTTP/1.1 #{code}\r\n"
         header.each_pair {|k,v|
           data << "#{k}: #{v}\r\n"
         }
-        data << "\r\n".freeze
+        data << "\r\n"
         @io.write(data)
 
         @io.write(body)
@@ -619,7 +619,7 @@ def send_response_nobody(code, header)
         header.each_pair {|k,v|
           data << "#{k}: #{v}\r\n"
         }
-        data << "\r\n".freeze
+        data << "\r\n"
         @io.write(data)
       end
 
diff --git a/lib/fluent/plugin/in_monitor_agent.rb b/lib/fluent/plugin/in_monitor_agent.rb
index 0654f7b7d2..22e9b3ffc0 100644
--- a/lib/fluent/plugin/in_monitor_agent.rb
+++ b/lib/fluent/plugin/in_monitor_agent.rb
@@ -125,21 +125,21 @@ def render_ltsv(obj, code: 200)
 
       def build_object(opts)
         qs = opts[:query]
-        if tag = qs['tag'.freeze].first
+        if tag = qs['tag'].first
           # ?tag= to search an output plugin by match pattern
           if obj = @agent.plugin_info_by_tag(tag, opts)
             list = [obj]
           else
             list = []
           end
-        elsif plugin_id = (qs['@id'.freeze].first || qs['id'.freeze].first)
+        elsif plugin_id = (qs['@id'].first || qs['id'].first)
           # ?@id= to search a plugin by 'id <plugin_id>' config param
           if obj = @agent.plugin_info_by_id(plugin_id, opts)
             list = [obj]
           else
             list = []
           end
-        elsif plugin_type = (qs['@type'.freeze].first || qs['type'.freeze].first)
+        elsif plugin_type = (qs['@type'].first || qs['type'].first)
           # ?@type= to search plugins by 'type <type>' config param
           list = @agent.plugins_info_by_type(plugin_type, opts)
         else
@@ -160,22 +160,22 @@ def build_option(req)
         # if ?debug=1 is set, set :with_debug_info for get_monitor_info
         # and :pretty_json for render_json_error
         opts = { query: qs }
-        if qs['debug'.freeze].first
+        if qs['debug'].first
           opts[:with_debug_info] = true
           opts[:pretty_json] = true
         end
 
-        if ivars = qs['with_ivars'.freeze].first
+        if ivars = qs['with_ivars'].first
           opts[:ivars] = ivars.split(',')
         end
 
-        if with_config = qs['with_config'.freeze].first
+        if with_config = qs['with_config'].first
           opts[:with_config] = Fluent::Config.bool_value(with_config)
         else
           opts[:with_config] = @agent.include_config
         end
 
-        if with_retry = qs['with_retry'.freeze].first
+        if with_retry = qs['with_retry'].first
           opts[:with_retry] = Fluent::Config.bool_value(with_retry)
         else
           opts[:with_retry] = @agent.include_retry
@@ -388,13 +388,13 @@ def get_retry_info(pe_retry)
     def plugin_category(pe)
       case pe
       when Fluent::Plugin::Input
-        'input'.freeze
+        'input'
       when Fluent::Plugin::Output, Fluent::Plugin::MultiOutput, Fluent::Plugin::BareOutput
-        'output'.freeze
+        'output'
       when Fluent::Plugin::Filter
-        'filter'.freeze
+        'filter'
       else
-        'unknown'.freeze
+        'unknown'
       end
     end
 
diff --git a/lib/fluent/plugin/in_syslog.rb b/lib/fluent/plugin/in_syslog.rb
index 133474b4c4..01fb734c7f 100644
--- a/lib/fluent/plugin/in_syslog.rb
+++ b/lib/fluent/plugin/in_syslog.rb
@@ -83,7 +83,7 @@ class SyslogInput < Input
     desc 'If true, add source host to event record.'
     config_param :include_source_host, :bool, default: false, deprecated: 'use "source_hostname_key" or "source_address_key" instead.'
     desc 'Specify key of source host when include_source_host is true.'
-    config_param :source_host_key, :string, default: 'source_host'.freeze
+    config_param :source_host_key, :string, default: 'source_host'
     desc 'Enable the option to emit unmatched lines.'
     config_param :emit_unmatched_lines, :bool, default: false
 
diff --git a/test/config/test_configurable.rb b/test/config/test_configurable.rb
index 4bd23e027e..0ce404a47e 100644
--- a/test/config/test_configurable.rb
+++ b/test/config/test_configurable.rb
@@ -535,7 +535,7 @@ class TestConfigurable < ::Test::Unit::TestCase
 
         test 'can accept frozen string' do
           b2 = ConfigurableSpec::Base2.new
-          assert_instance_of(ConfigurableSpec::Base2, b2.configure(config_element("", "", {"name1" => "t1".freeze, "name5" => "t5", "opt3" => "a"})))
+          assert_instance_of(ConfigurableSpec::Base2, b2.configure(config_element("", "", {"name1" => "t1", "name5" => "t5", "opt3" => "a"})))
         end
 
         test 'raise errors without any specifications for param without defaults' do