diff --git a/apps/server/src/domains/room/RoomManager.ts b/apps/server/src/domains/room/RoomManager.ts
index ff3bb62..7dc2152 100644
--- a/apps/server/src/domains/room/RoomManager.ts
+++ b/apps/server/src/domains/room/RoomManager.ts
@@ -6,6 +6,7 @@
 import { RoomJoinService } from "./application/services/RoomJoinService";
 import { RoomExitService } from "./application/services/RoomExitService";
 import { RoomQueryService } from "./application/services/RoomQueryService";
+import type { JoinRoomResult } from "./application/ports/roomUseCasePorts";
 
 /** ルーム操作の公開インターフェースを提供するマネージャ */
 export class RoomManager {
@@ -21,7 +22,7 @@
   }
 
   // ルームにプレイヤーを追加する，ルームが未作成なら新規作成する
-  public addPlayerToRoom(roomId: string, socketId: string, playerName: string): roomTypes.Room {
+  public addPlayerToRoom(roomId: string, socketId: string, playerName: string): JoinRoomResult {
     return this.roomJoinService.addPlayerToRoom(roomId, socketId, playerName);
   }
 
diff --git a/apps/server/src/domains/room/application/ports/roomUseCasePorts.ts b/apps/server/src/domains/room/application/ports/roomUseCasePorts.ts
index c3e9cb8..b00b3c8 100644
--- a/apps/server/src/domains/room/application/ports/roomUseCasePorts.ts
+++ b/apps/server/src/domains/room/application/ports/roomUseCasePorts.ts
@@ -4,9 +4,15 @@
  */
 import type { roomTypes } from "@repo/shared";
 
+/** ルーム参加処理の実行結果 */
+export type JoinRoomResult = {
+  room: roomTypes.Room;
+  status: "joined" | "duplicate" | "full";
+};
+
 /** ルーム参加ユースケースが利用する参加操作ポート */
 export interface JoinRoomPort {
-  addPlayerToRoom(roomId: string, socketId: string, playerName: string): roomTypes.Room;
+  addPlayerToRoom(roomId: string, socketId: string, playerName: string): JoinRoomResult;
 }
 
 /** ルーム切断ユースケースが利用する退出操作ポート */
diff --git a/apps/server/src/domains/room/application/services/RoomJoinService.ts b/apps/server/src/domains/room/application/services/RoomJoinService.ts
index 4af2108..d096dcc 100644
--- a/apps/server/src/domains/room/application/services/RoomJoinService.ts
+++ b/apps/server/src/domains/room/application/services/RoomJoinService.ts
@@ -5,12 +5,13 @@
 import { config, roomConsts } from "@repo/shared";
 import type { roomTypes } from "@repo/shared";
 import { logEvent } from "@server/logging/logEvent";
+import type { JoinRoomResult } from "../ports/roomUseCasePorts";
 
 /** 参加要求に応じてルーム作成と参加者追加を行うサービス */
 export class RoomJoinService {
   constructor(private rooms: Map<string, roomTypes.Room>) {}
 
-  public addPlayerToRoom(roomId: string, socketId: string, playerName: string): roomTypes.Room {
+  public addPlayerToRoom(roomId: string, socketId: string, playerName: string): JoinRoomResult {
     let room = this.rooms.get(roomId);
     if (!room) {
       room = {
@@ -30,6 +31,32 @@
       });
     }
 
+    // 同一ソケットの重複参加を防止する
+    const alreadyJoined = room.players.some((player) => player.id === socketId);
+    if (alreadyJoined) {
+      logEvent("RoomJoinService", {
+        event: "PLAYER_JOIN",
+        result: "ignored_duplicate",
+        roomId,
+        socketId,
+        totalPlayers: room.players.length,
+      });
+      return { room, status: "duplicate" };
+    }
+
+    // ルーム満員時の参加を拒否する
+    if (room.players.length >= room.maxPlayers) {
+      logEvent("RoomJoinService", {
+        event: "PLAYER_JOIN",
+        result: "ignored_room_full",
+        roomId,
+        socketId,
+        maxPlayers: room.maxPlayers,
+        totalPlayers: room.players.length,
+      });
+      return { room, status: "full" };
+    }
+
     const newPlayer: roomTypes.RoomMember = {
       id: socketId,
       name: playerName,
@@ -47,6 +74,6 @@
       totalPlayers: room.players.length,
     });
 
-    return room;
+    return { room, status: "joined" };
   }
 }
diff --git a/apps/server/src/domains/room/application/useCases/joinRoomUseCase.ts b/apps/server/src/domains/room/application/useCases/joinRoomUseCase.ts
index 661618c..e432db4 100644
--- a/apps/server/src/domains/room/application/useCases/joinRoomUseCase.ts
+++ b/apps/server/src/domains/room/application/useCases/joinRoomUseCase.ts
@@ -3,7 +3,7 @@
  * ルーム参加要求を処理し，状態更新を配信するユースケース
  */
 import type { roomTypes } from "@repo/shared";
-import type { JoinRoomPort } from "../ports/roomUseCasePorts";
+import type { JoinRoomPort, JoinRoomResult } from "../ports/roomUseCasePorts";
 import { logEvent } from "@server/logging/logEvent";
 
 type JoinRoomUseCaseParams = {
@@ -19,7 +19,7 @@
   socketId,
   data,
   publishRoomUpdate,
-}: JoinRoomUseCaseParams) => {
+}: JoinRoomUseCaseParams): JoinRoomResult => {
   const { roomId, playerName } = data;
   logEvent("RoomUseCase", {
     event: "JOIN_ROOM",
@@ -29,15 +29,27 @@
     playerName,
   });
 
-  const room = roomManager.addPlayerToRoom(roomId, socketId, playerName);
+  const joinResult = roomManager.addPlayerToRoom(roomId, socketId, playerName);
+  if (joinResult.status !== "joined") {
+    logEvent("RoomUseCase", {
+      event: "JOIN_ROOM",
+      result: "rejected",
+      reason: joinResult.status,
+      roomId,
+      socketId,
+    });
+    return joinResult;
+  }
 
-  publishRoomUpdate(roomId, room);
+  publishRoomUpdate(roomId, joinResult.room);
   logEvent("RoomUseCase", {
     event: "ROOM_UPDATE",
     result: "emitted",
     roomId,
     socketId,
-    ownerId: room.ownerId,
-    totalPlayers: room.players.length,
+    ownerId: joinResult.room.ownerId,
+    totalPlayers: joinResult.room.players.length,
   });
+
+  return joinResult;
 };
diff --git a/apps/server/src/network/handlers/room/registerRoomHandlers.ts b/apps/server/src/network/handlers/room/registerRoomHandlers.ts
index 0c3dd03..7336761 100644
--- a/apps/server/src/network/handlers/room/registerRoomHandlers.ts
+++ b/apps/server/src/network/handlers/room/registerRoomHandlers.ts
@@ -36,11 +36,22 @@
 
     socket.join(roomId);
 
-    joinRoomUseCase({
+    const joinResult = joinRoomUseCase({
       roomManager,
       socketId: socket.id,
       data,
       publishRoomUpdate: roomPublisher.publishRoomUpdate,
     });
+
+    // 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);
+      logEvent("Network", {
+        event: "JOIN_ROOM",
+        result: "rejected_room_full",
+        roomId,
+        socketId: socket.id,
+      });
+    }
   });
 };
diff --git a/packages/shared/src/config/gameConfig.ts b/packages/shared/src/config/gameConfig.ts
index 32b91e2..bc2a755 100644
--- a/packages/shared/src/config/gameConfig.ts
+++ b/packages/shared/src/config/gameConfig.ts
@@ -1,6 +1,6 @@
 export const GAME_CONFIG = {
   // ゲーム設定
-  MAX_PLAYERS_PER_ROOM: 4,  // ルーム収容人数設定
+  MAX_PLAYERS_PER_ROOM: 100,  // ルーム収容人数設定
   GAME_DURATION_SEC: 180, // 1ゲームの制限時間（3分 = 180秒）
 
   // UI表示更新設定