diff --git a/apps/client/src/app.tsx b/apps/client/src/app.tsx
index 5c6272a..08b1b54 100644
--- a/apps/client/src/app.tsx
+++ b/apps/client/src/app.tsx
@@ -9,11 +9,16 @@
 import { appConsts } from "@repo/shared";
 
 export default function App() {
-  const { scenePhase, room, myId } = useAppFlow();
+  const { scenePhase, room, myId, joinErrorMessage } = useAppFlow();
 
   // タイトル画面分岐
   if (scenePhase === appConsts.ScenePhase.TITLE) {
-    return <TitleScene onJoin={(payload) => socketManager.title.joinRoom(payload)} />;
+    return (
+      <TitleScene
+        onJoin={(payload) => socketManager.title.joinRoom(payload)}
+        joinErrorMessage={joinErrorMessage}
+      />
+    );
   }
   
   // ロビー画面分岐
diff --git a/apps/client/src/hooks/useAppFlow.ts b/apps/client/src/hooks/useAppFlow.ts
index b68228b..6fe6ae8 100644
--- a/apps/client/src/hooks/useAppFlow.ts
+++ b/apps/client/src/hooks/useAppFlow.ts
@@ -7,12 +7,14 @@
   scenePhase: appTypes.ScenePhase;
   room: roomTypes.Room | null;
   myId: string | null;
+  joinErrorMessage: string | null;
 };
 
 export const useAppFlow = (): AppFlowState => {
   const [scenePhase, setScenePhase] = useState<appTypes.ScenePhase>(appConsts.ScenePhase.TITLE);
   const [room, setRoom] = useState<roomTypes.Room | null>(null);
   const [myId, setMyId] = useState<string | null>(null);
+  const [joinErrorMessage, setJoinErrorMessage] = useState<string | null>(null);
 
   useEffect(() => {
     const handleConnect = (id: string) => {
@@ -20,22 +22,30 @@
     };
     const handleRoomUpdate = (updatedRoom: roomTypes.Room) => {
       setRoom(updatedRoom);
+      setJoinErrorMessage(null);
       setScenePhase(appConsts.ScenePhase.LOBBY);
     };
+    const handleJoinRejected = (payload: roomTypes.JoinRoomRejectedPayload) => {
+      if (payload.reason === "full") {
+        setJoinErrorMessage(`ルーム ${payload.roomId} は満員です`);
+      }
+    };
     const handleGameStart = () => {
       setScenePhase(appConsts.ScenePhase.PLAYING);
     };
 
     socketManager.common.onConnect(handleConnect);
+    socketManager.title.onJoinRejected(handleJoinRejected);
     socketManager.lobby.onRoomUpdate(handleRoomUpdate);
     socketManager.game.onGameStart(handleGameStart);
 
     return () => {
       socketManager.common.offConnect(handleConnect);
+      socketManager.title.offJoinRejected(handleJoinRejected);
       socketManager.lobby.offRoomUpdate(handleRoomUpdate);
       socketManager.game.offGameStart(handleGameStart);
     };
   }, []);
 
-  return { scenePhase, room, myId };
+  return { scenePhase, room, myId, joinErrorMessage };
 };
diff --git a/apps/client/src/network/handlers/TitleHandler.ts b/apps/client/src/network/handlers/TitleHandler.ts
index 32248a8..3246817 100644
--- a/apps/client/src/network/handlers/TitleHandler.ts
+++ b/apps/client/src/network/handlers/TitleHandler.ts
@@ -4,12 +4,20 @@
 
 type TitleHandler = {
   joinRoom: (payload: roomTypes.JoinRoomPayload) => void;
+  onJoinRejected: (callback: (payload: roomTypes.JoinRoomRejectedPayload) => void) => void;
+  offJoinRejected: (callback: (payload: roomTypes.JoinRoomRejectedPayload) => void) => void;
 };
 
 export const createTitleHandler = (socket: Socket): TitleHandler => {
   return {
     joinRoom: (payload: roomTypes.JoinRoomPayload) => {
       socket.emit(protocol.SocketEvents.JOIN_ROOM, payload);
+    },
+    onJoinRejected: (callback: (payload: roomTypes.JoinRoomRejectedPayload) => void) => {
+      socket.on(protocol.SocketEvents.ROOM_JOIN_REJECTED, callback);
+    },
+    offJoinRejected: (callback: (payload: roomTypes.JoinRoomRejectedPayload) => void) => {
+      socket.off(protocol.SocketEvents.ROOM_JOIN_REJECTED, callback);
     }
   };
 };
diff --git a/apps/client/src/scenes/title/TitleScene.tsx b/apps/client/src/scenes/title/TitleScene.tsx
index d475c08..cec2fdc 100644
--- a/apps/client/src/scenes/title/TitleScene.tsx
+++ b/apps/client/src/scenes/title/TitleScene.tsx
@@ -5,9 +5,11 @@
 type Props = {
   // 入室実行時呼び出しコールバック
   onJoin: (payload: roomTypes.JoinRoomPayload) => void;
+  // 入室失敗時の表示メッセージ
+  joinErrorMessage: string | null;
 };
 
-export const TitleScene = ({ onJoin }: Props) => {
+export const TitleScene = ({ onJoin, joinErrorMessage }: Props) => {
   // プレイヤー名入力値
   const [playerName, setPlayerName] = useState("");
   // ルームID入力値
@@ -59,6 +61,10 @@
       >
         ルームに入る / 作る
       </button>
+
+      {joinErrorMessage && (
+        <p style={{ marginTop: "14px", color: "#f87171", fontWeight: "bold" }}>{joinErrorMessage}</p>
+      )}
     </div>
   );
 };
\ No newline at end of file
diff --git a/apps/server/src/network/handlers/room/createRoomEventPublisher.ts b/apps/server/src/network/handlers/room/createRoomEventPublisher.ts
index 3184664..5e2bf4d 100644
--- a/apps/server/src/network/handlers/room/createRoomEventPublisher.ts
+++ b/apps/server/src/network/handlers/room/createRoomEventPublisher.ts
@@ -14,6 +14,7 @@
 /** ルーム更新イベントの送信インターフェース */
 export type RoomEventPublisher = {
   publishRoomUpdate: (roomId: RoomId, room: RoomUpdatePayload) => void;
+  publishJoinRejected: (payload: roomTypes.JoinRoomRejectedPayload) => void;
 };
 
 /** 共通送信コンテキストからルームイベント送信関数を生成する */
@@ -24,6 +25,9 @@
     publishRoomUpdate: (roomId: RoomId, room: RoomUpdatePayload) => {
       common.emitToRoom(roomId, protocol.SocketEvents.ROOM_UPDATE, room);
     },
+    publishJoinRejected: (payload: roomTypes.JoinRoomRejectedPayload) => {
+      common.emitToSocket(protocol.SocketEvents.ROOM_JOIN_REJECTED, payload);
+    },
   };
 };
 
@@ -35,5 +39,8 @@
     publishRoomUpdate: (roomId: RoomId, room: RoomUpdatePayload) => {
       emitToRoom(roomId, protocol.SocketEvents.ROOM_UPDATE, room);
     },
+    publishJoinRejected: () => {
+      return;
+    },
   };
 };
diff --git a/apps/server/src/network/handlers/room/registerRoomHandlers.ts b/apps/server/src/network/handlers/room/registerRoomHandlers.ts
index 7336761..33dd81c 100644
--- a/apps/server/src/network/handlers/room/registerRoomHandlers.ts
+++ b/apps/server/src/network/handlers/room/registerRoomHandlers.ts
@@ -46,6 +46,10 @@
     // 満員拒否時はソケットのルーム参加状態を巻き戻す
     if (joinResult.status === "full") {
       socket.leave(roomId);
+      roomPublisher.publishJoinRejected({
+        roomId,
+        reason: "full",
+      });
       logEvent("Network", {
         event: "JOIN_ROOM",
         result: "rejected_room_full",
diff --git a/packages/shared/src/domains/room/room.type.ts b/packages/shared/src/domains/room/room.type.ts
index 4c1ac67..6226444 100644
--- a/packages/shared/src/domains/room/room.type.ts
+++ b/packages/shared/src/domains/room/room.type.ts
@@ -22,4 +22,13 @@
 export interface JoinRoomPayload {
   roomId: string;
   playerName: string;
+}
+
+// ルーム参加拒否理由型
+export type JoinRoomRejectedReason = "full";
+
+// ルーム参加拒否通知ペイロード型
+export interface JoinRoomRejectedPayload {
+  roomId: string;
+  reason: JoinRoomRejectedReason;
 }
\ No newline at end of file
diff --git a/packages/shared/src/protocol/events.ts b/packages/shared/src/protocol/events.ts
index 5e90344..39a1c4a 100644
--- a/packages/shared/src/protocol/events.ts
+++ b/packages/shared/src/protocol/events.ts
@@ -5,6 +5,7 @@
   
   // ロビー・ルーム関連イベント名
   JOIN_ROOM: "join-room",
+  ROOM_JOIN_REJECTED: "room-join-rejected",
   ROOM_UPDATE: "room-update",
   START_GAME: "start-game",
   GAME_START: "game-start",