diff --git a/core/error.ts b/core/error.ts
index 7f47132..9ca25db 100644
--- a/core/error.ts
+++ b/core/error.ts
@@ -46,8 +46,12 @@ export const errors = {
     errorCode: 105,
     message: "during the transition step",
   },
+  GAME_ENDED: {
+    errorCode: 107,
+    message: "game has ended",
+  },
   INVALID_STREAM_QUERY: {
-    errorCode: 106,
+    errorCode: 108,
     message: "invalid query parameter",
   },
   INVALID_SCREEN_NAME: {
diff --git a/v1/_matches.ts b/v1/_matches.ts
index 416dd7a..690eb0e 100644
--- a/v1/_matches.ts
+++ b/v1/_matches.ts
@@ -197,6 +197,10 @@ router.patch(
       throw new ServerError(errors.DURING_TRANSITION_STEP);
     }
 
+    if (game.isEnded()) {
+      throw new ServerError(errors.GAME_ENDED);
+    }
+
     const actionData = ctx.get("data");
     const isValid = validator.validateRequestBody(
       actionData,
diff --git a/v1/test/action_after_game_end_test.ts b/v1/test/action_after_game_end_test.ts
new file mode 100644
index 0000000..c49ee12
--- /dev/null
+++ b/v1/test/action_after_game_end_test.ts
@@ -0,0 +1,125 @@
+import { assert, assertEquals } from "@std/assert";
+import { FakeTime } from "@std/testing/time";
+
+import ApiClient from "../../client/client.ts";
+import { errors } from "../../core/error.ts";
+import { validator } from "../parts/openapi.ts";
+
+const ac = new ApiClient();
+
+function sleep(ms: number) {
+  return new Promise((resolve) => setTimeout(resolve, ms));
+}
+
+Deno.test({
+  name: "action_after_game_end_test",
+  fn: async (t) => {
+    using time = new FakeTime();
+
+    let gameId: string;
+    let pic1: string;
+    let pic2: string;
+
+    await t.step("create game", async () => {
+      const res = await ac.createMatch({ 
+        name: "test-game-end", 
+        boardName: "A-1",
+        totalTurn: 2  // Very short game for quick testing
+      });
+      if (res.success === false) {
+        throw Error("Response Error. ErrorCode:" + res.data.errorCode);
+      }
+      
+      gameId = res.data.id;
+      assert(gameId);
+    });
+
+    await t.step("join players", async () => {
+      const player1Res = await ac.addPlayer(gameId, { });
+      const player2Res = await ac.addPlayer(gameId, { });
+      
+      if (player1Res.success === false) {
+        throw Error("Response Error. ErrorCode:" + player1Res.data.errorCode);
+      }
+      if (player2Res.success === false) {
+        throw Error("Response Error. ErrorCode:" + player2Res.data.errorCode);
+      }
+      
+      pic1 = player1Res.data.pic;
+      pic2 = player2Res.data.pic;
+      assert(pic1);
+      assert(pic2);
+    });
+
+    await t.step("start game", async () => {
+      const res = await ac.startMatch(gameId);
+      if (res.success === false) {
+        throw Error("Response Error. ErrorCode:" + res.data.errorCode);
+      }
+    });
+
+    await t.step("play turns until game ends", async () => {
+      // Get game info to understand timing
+      let gameInfo = await ac.getMatch(gameId);
+      if (gameInfo.success === false) {
+        throw Error("Response Error. ErrorCode:" + gameInfo.data.errorCode);
+      }
+
+      const operationSec = gameInfo.data.operationSec || 3;
+      const transitionSec = gameInfo.data.transitionSec || 1;
+      const totalTurn = gameInfo.data.totalTurn || 2;
+      
+      // Play for all turns until game ends
+      for (let turn = 1; turn <= totalTurn; turn++) {
+        // Fast forward to the start of the turn
+        const turnStartTime = gameInfo.data.startedAtUnixTime! + 
+          (turn - 1) * (operationSec + transitionSec);
+        time.tick(turnStartTime * 1000 - Date.now());
+        
+        // Send actions during the turn
+        await ac.setAction(gameId, {
+          actions: [{ agentId: 0, type: "PUT", x: 0, y: turn - 1 }],
+        }, pic1);
+        
+        // Wait for turn to complete
+        const turnEndTime = turnStartTime + operationSec;
+        time.tick(turnEndTime * 1000 - Date.now() + 100);
+        
+        // Wait for transition to complete
+        const transitionEndTime = turnEndTime + transitionSec;
+        time.tick(transitionEndTime * 1000 - Date.now() + 100);
+      }
+      
+      // Ensure game has ended
+      gameInfo = await ac.getMatch(gameId);
+      if (gameInfo.success === false) {
+        throw Error("Response Error. ErrorCode:" + gameInfo.data.errorCode);
+      }
+      
+      assertEquals(gameInfo.data.gaming, false);
+      assertEquals(gameInfo.data.ending, true);
+    });
+
+    await t.step("send action after game end should fail", async () => {
+      const actionRes = await ac.setAction(gameId, {
+        actions: [{ agentId: 0, type: "PUT", x: 1, y: 1 }],
+      }, pic1);
+      
+      // This should fail with GAME_ENDED error
+      assertEquals(actionRes.success, false);
+      if (actionRes.success === false) {
+        assertEquals(actionRes.data.errorCode, errors.GAME_ENDED.errorCode);
+        assertEquals(actionRes.data.message, errors.GAME_ENDED.message);
+      }
+      
+      // Validate response format
+      assert(validator.validateResponse(
+        actionRes.data,
+        "/matches/{gameId}/actions",
+        "patch",
+        "400",
+        "application/json",
+      ));
+    });
+  },
+});
\ No newline at end of file