diff --git a/graphql/schema.graphql b/graphql/schema.graphql
index f1c7c40..6b77359 100644
--- a/graphql/schema.graphql
+++ b/graphql/schema.graphql
@@ -27,6 +27,7 @@ type Node {
   name: String!
   groupId: ID
   domain: String    # 所属しているdomain
+  expiresAt: AWSDateTime
   heartbeatIntervalSeconds: Int
 }
 
diff --git a/js/functions/joinGroupFunction.js b/js/functions/joinGroupFunction.js
index d889496..72871e1 100644
--- a/js/functions/joinGroupFunction.js
+++ b/js/functions/joinGroupFunction.js
@@ -64,12 +64,14 @@ export function response(ctx) {
   // TransactWriteItemsは個別のアイテムを返さないため、
   // リクエストパラメータからNode型を構築
   const { groupId, domain, nodeId } = ctx.args;
+  const group = ctx.stash.group;
 
   return {
     id: nodeId,
     name: `Node ${nodeId}`,
     groupId: groupId,
     domain: domain,
+    expiresAt: group ? group.expiresAt : null,
     heartbeatIntervalSeconds: +(ctx.env.MESH_MEMBER_HEARTBEAT_INTERVAL_SECONDS || '120')
   };
 }
diff --git a/spec/fixtures/mutations/join_group.graphql b/spec/fixtures/mutations/join_group.graphql
index f67b1f3..de415ba 100644
--- a/spec/fixtures/mutations/join_group.graphql
+++ b/spec/fixtures/mutations/join_group.graphql
@@ -4,6 +4,7 @@ mutation JoinGroup($groupId: ID!, $domain: String!, $nodeId: ID!) {
     name
     groupId
     domain
+    expiresAt
     heartbeatIntervalSeconds
   }
 }
diff --git a/spec/requests/join_group_expires_at_spec.rb b/spec/requests/join_group_expires_at_spec.rb
new file mode 100644
index 0000000..a946cf1
--- /dev/null
+++ b/spec/requests/join_group_expires_at_spec.rb
@@ -0,0 +1,33 @@
+require "spec_helper"
+require "time"
+
+RSpec.describe "joinGroup expiresAt validation", type: :request do
+  let(:timestamp) { (Time.now.to_f * 1000).to_i }
+  let(:domain) { "test-join-expires-at-#{timestamp}.example.com" }
+  let(:host_id) { "host-#{timestamp}" }
+  let(:node_id) { "node-#{timestamp}" }
+  let(:group_name) { "Join Group expiresAt Test" }
+
+  it "joinGroup mutation returns expiresAt" do
+    # 1. グループを作成
+    group = create_test_group(group_name, host_id, domain)
+    group_id = group["id"]
+    group_expires_at = group["expiresAt"]
+    expect(group_id).not_to be_nil
+    expect(group_expires_at).to match_iso8601
+
+    # 2. グループに参加
+    join_response = join_test_node(group_id, domain, node_id)
+
+    # 3. expiresAtが返されることを確認
+    expect(join_response["expiresAt"]).not_to be_nil
+    expect(join_response["expiresAt"]).to match_iso8601
+
+    # 4. グループのexpiresAtと一致することを確認
+    # AWSDateTimeのフォーマットが微妙に異なる可能性（ミリ秒の有無など）を考慮し、
+    # Timeオブジェクトに変換して比較する
+    group_time = Time.parse(group_expires_at)
+    join_time = Time.parse(join_response["expiresAt"])
+    expect(join_time).to eq(group_time)
+  end
+end