diff --git a/apps/client/src/app.tsx b/apps/client/src/app.tsx
index 832b480..37a01a3 100644
--- a/apps/client/src/app.tsx
+++ b/apps/client/src/app.tsx
@@ -41,7 +41,9 @@
       <LobbyScene
         room={room}
         myId={myId}
-        onStart={() => socketManager.lobby.startGame()}
+        onStart={(targetPlayerCount) =>
+          socketManager.lobby.startGame(targetPlayerCount)
+        }
         onBackToTitle={() => returnToTitle({ leaveRoom: true })}
       />
     );
diff --git a/apps/client/src/network/handlers/LobbyHandler.ts b/apps/client/src/network/handlers/LobbyHandler.ts
index ca0cb41..6e9c994 100644
--- a/apps/client/src/network/handlers/LobbyHandler.ts
+++ b/apps/client/src/network/handlers/LobbyHandler.ts
@@ -10,15 +10,28 @@
 
 /** ロビー画面で利用する通信操作の契約 */
 type LobbyHandler = {
-  onRoomUpdate: (callback: (room: ServerToClientPayloadOf<typeof protocol.SocketEvents.ROOM_UPDATE>) => void) => void;
-  onceRoomUpdate: (callback: (room: ServerToClientPayloadOf<typeof protocol.SocketEvents.ROOM_UPDATE>) => void) => void;
-  offRoomUpdate: (callback: (room: ServerToClientPayloadOf<typeof protocol.SocketEvents.ROOM_UPDATE>) => void) => void;
-  startGame: () => void;
+  onRoomUpdate: (
+    callback: (
+      room: ServerToClientPayloadOf<typeof protocol.SocketEvents.ROOM_UPDATE>,
+    ) => void,
+  ) => void;
+  onceRoomUpdate: (
+    callback: (
+      room: ServerToClientPayloadOf<typeof protocol.SocketEvents.ROOM_UPDATE>,
+    ) => void,
+  ) => void;
+  offRoomUpdate: (
+    callback: (
+      room: ServerToClientPayloadOf<typeof protocol.SocketEvents.ROOM_UPDATE>,
+    ) => void,
+  ) => void;
+  startGame: (targetPlayerCount?: number) => void;
 };
 
 /** ロビー画面向けのソケットハンドラを生成する */
 export const createLobbyHandler = (socket: Socket): LobbyHandler => {
-  const { onEvent, onceEvent, offEvent, emitEvent } = createClientSocketEventBridge(socket);
+  const { onEvent, onceEvent, offEvent, emitEvent } =
+    createClientSocketEventBridge(socket);
 
   return {
     onRoomUpdate: (callback) => {
@@ -30,9 +43,11 @@
     offRoomUpdate: (callback) => {
       offEvent(protocol.SocketEvents.ROOM_UPDATE, callback);
     },
-    startGame: () => {
-      emitEvent(protocol.SocketEvents.START_GAME);
-    }
+    startGame: (targetPlayerCount) => {
+      emitEvent(protocol.SocketEvents.START_GAME, {
+        targetPlayerCount,
+      });
+    },
   };
 };
 
diff --git a/apps/client/src/scenes/lobby/LobbyScene.tsx b/apps/client/src/scenes/lobby/LobbyScene.tsx
index 7f353e0..12ae64a 100644
--- a/apps/client/src/scenes/lobby/LobbyScene.tsx
+++ b/apps/client/src/scenes/lobby/LobbyScene.tsx
@@ -1,9 +1,10 @@
+import { useEffect, useMemo, useState } from "react";
 import type { roomTypes } from "@repo/shared";
 
 type Props = {
   room: roomTypes.Room | null;
   myId: string | null;
-  onStart: () => void;
+  onStart: (targetPlayerCount: number) => void;
   onBackToTitle: () => void;
 };
 
@@ -12,6 +13,55 @@
     return <div style={{ color: "white", padding: 40 }}>読み込み中...</div>;
 
   const isMeOwner = room.ownerId === myId;
+  const teamUnit = 4;
+  const minimumStartPlayerCount = useMemo(() => {
+    return Math.max(
+      teamUnit,
+      Math.ceil(room.players.length / teamUnit) * teamUnit,
+    );
+  }, [room.players.length]);
+
+  const maxStartPlayerCount = useMemo(() => {
+    return Math.max(
+      minimumStartPlayerCount,
+      Math.floor(room.maxPlayers / teamUnit) * teamUnit,
+    );
+  }, [minimumStartPlayerCount, room.maxPlayers]);
+
+  const startPlayerCountOptions = useMemo(() => {
+    const options: number[] = [];
+    for (
+      let count = minimumStartPlayerCount;
+      count <= maxStartPlayerCount;
+      count += teamUnit
+    ) {
+      options.push(count);
+    }
+
+    return options;
+  }, [minimumStartPlayerCount, maxStartPlayerCount]);
+
+  const [selectedStartPlayerCount, setSelectedStartPlayerCount] = useState(
+    minimumStartPlayerCount,
+  );
+
+  useEffect(() => {
+    setSelectedStartPlayerCount((prev) => {
+      if (prev < minimumStartPlayerCount || prev > maxStartPlayerCount) {
+        return minimumStartPlayerCount;
+      }
+
+      if (prev % teamUnit !== 0) {
+        return minimumStartPlayerCount;
+      }
+
+      return prev;
+    });
+  }, [minimumStartPlayerCount, maxStartPlayerCount]);
+
+  const handleStart = () => {
+    onStart(selectedStartPlayerCount);
+  };
 
   return (
     <>
@@ -111,24 +161,67 @@
             }}
           >
             {isMeOwner ? (
-              <button
-                onClick={onStart}
+              <div
                 style={{
                   width: "100%",
                   maxWidth: "350px",
-                  padding: "20px",
-                  fontSize: "clamp(1.2rem, 3vw, 1.8rem)",
-                  cursor: "pointer",
-                  backgroundColor: "#4ade80",
-                  color: "#111",
-                  border: "none",
-                  borderRadius: "8px",
-                  fontWeight: "bold",
-                  boxShadow: "0 4px 6px rgba(0,0,0,0.3)",
+                  display: "flex",
+                  flexDirection: "column",
+                  gap: "12px",
                 }}
               >
-                ゲームスタート
-              </button>
+                <label
+                  htmlFor="start-player-count"
+                  style={{
+                    fontSize: "0.95rem",
+                    fontWeight: 700,
+                    textShadow: "1px 1px 2px rgba(0,0,0,0.7)",
+                  }}
+                >
+                  ゲーム人数
+                </label>
+                <select
+                  id="start-player-count"
+                  value={selectedStartPlayerCount}
+                  onChange={(event) => {
+                    setSelectedStartPlayerCount(Number(event.target.value));
+                  }}
+                  style={{
+                    width: "100%",
+                    padding: "10px 12px",
+                    borderRadius: "8px",
+                    border: "1px solid rgba(255,255,255,0.4)",
+                    background: "rgba(0,0,0,0.55)",
+                    color: "white",
+                    fontSize: "1rem",
+                    fontWeight: 700,
+                  }}
+                >
+                  {startPlayerCountOptions.map((count) => (
+                    <option key={count} value={count}>
+                      {count}人
+                    </option>
+                  ))}
+                </select>
+
+                <button
+                  onClick={handleStart}
+                  style={{
+                    width: "100%",
+                    padding: "20px",
+                    fontSize: "clamp(1.2rem, 3vw, 1.8rem)",
+                    cursor: "pointer",
+                    backgroundColor: "#4ade80",
+                    color: "#111",
+                    border: "none",
+                    borderRadius: "8px",
+                    fontWeight: "bold",
+                    boxShadow: "0 4px 6px rgba(0,0,0,0.3)",
+                  }}
+                >
+                  ゲームスタート
+                </button>
+              </div>
             ) : (
               <div
                 style={{
diff --git a/apps/server/src/application/coordinators/startGameCoordinator.ts b/apps/server/src/application/coordinators/startGameCoordinator.ts
index 5018aca..6df458b 100644
--- a/apps/server/src/application/coordinators/startGameCoordinator.ts
+++ b/apps/server/src/application/coordinators/startGameCoordinator.ts
@@ -18,6 +18,7 @@
 
 type StartGameCoordinatorParams = {
   ownerId: string;
+  requestedPlayerCount?: number;
 } & StartGameCoordinatorDeps & {
     output: StartGameOutputPort;
   };
@@ -25,6 +26,7 @@
 /** START_GAME受信時にルーム状態遷移を判定し，ゲーム開始ユースケースを実行する */
 export const startGameCoordinator = ({
   ownerId,
+  requestedPlayerCount,
   roomManager,
   runtimeRegistry,
   output,
@@ -80,6 +82,7 @@
   const sessionPlayerIds = createBalancedSessionPlayerIds(
     updatedRoom.roomId,
     humanPlayerIds,
+    requestedPlayerCount,
   );
   sessionPlayerIds.forEach((playerId) => {
     if (!isBotPlayerId(playerId)) {
diff --git a/apps/server/src/domains/game/application/services/BotRosterService.ts b/apps/server/src/domains/game/application/services/BotRosterService.ts
index 2c4e953..2073868 100644
--- a/apps/server/src/domains/game/application/services/BotRosterService.ts
+++ b/apps/server/src/domains/game/application/services/BotRosterService.ts
@@ -22,18 +22,50 @@
   return playerId.startsWith(BOT_PLAYER_ID_PREFIX);
 };
 
-const getRequiredBotCount = (humanPlayerCount: number): number => {
+const getMinimumTotalPlayers = (humanPlayerCount: number): number => {
   const teamCount = config.GAME_CONFIG.TEAM_COUNT;
   if (teamCount <= 0) {
-    return 0;
+    return humanPlayerCount;
   }
 
   const remainder = humanPlayerCount % teamCount;
   if (remainder === 0) {
-    return 0;
+    return humanPlayerCount;
   }
 
-  return teamCount - remainder;
+  return humanPlayerCount + (teamCount - remainder);
+};
+
+const resolveTargetTotalPlayers = (
+  humanPlayerCount: number,
+  requestedPlayerCount: number | undefined,
+): number => {
+  const minimumTotal = getMinimumTotalPlayers(humanPlayerCount);
+  const teamCount = config.GAME_CONFIG.TEAM_COUNT;
+  const maxTotal = config.GAME_CONFIG.MAX_PLAYERS_PER_ROOM;
+
+  if (
+    requestedPlayerCount === undefined ||
+    teamCount <= 0 ||
+    requestedPlayerCount > maxTotal ||
+    requestedPlayerCount < minimumTotal ||
+    requestedPlayerCount % teamCount !== 0
+  ) {
+    return minimumTotal;
+  }
+
+  return requestedPlayerCount;
+};
+
+const getRequiredBotCount = (
+  humanPlayerCount: number,
+  requestedPlayerCount: number | undefined,
+): number => {
+  const totalPlayers = resolveTargetTotalPlayers(
+    humanPlayerCount,
+    requestedPlayerCount,
+  );
+  return Math.max(0, totalPlayers - humanPlayerCount);
 };
 
 /**
@@ -42,8 +74,12 @@
 export const createBalancedSessionPlayerIds = (
   roomId: string,
   humanPlayerIds: string[],
+  requestedPlayerCount?: number,
 ): string[] => {
-  const requiredBotCount = getRequiredBotCount(humanPlayerIds.length);
+  const requiredBotCount = getRequiredBotCount(
+    humanPlayerIds.length,
+    requestedPlayerCount,
+  );
   if (requiredBotCount === 0) {
     return [...humanPlayerIds];
   }
diff --git a/apps/server/src/network/handlers/game/registerGameHandlers.ts b/apps/server/src/network/handlers/game/registerGameHandlers.ts
index bcc0631..4934d91 100644
--- a/apps/server/src/network/handlers/game/registerGameHandlers.ts
+++ b/apps/server/src/network/handlers/game/registerGameHandlers.ts
@@ -24,6 +24,7 @@
   isMovePayload,
   isPingPayload,
   isPlaceBombPayload,
+  isStartGamePayload,
 } from "@server/network/validation/socketPayloadValidators";
 import { createServerSocketOnBridge } from "@server/network/handlers/socketEventBridge";
 import { createPayloadGuard } from "@server/network/handlers/payloadGuard";
@@ -41,8 +42,10 @@
 export const registerGameHandlers = (
   io: Server,
   socket: Socket,
-  roomManager: FindRoomByOwnerPort & FindRoomByPlayerPort & RoomPhaseTransitionPort,
-  runtimeRegistry: FindGameByRoomPort & FindGameByPlayerPort
+  roomManager: FindRoomByOwnerPort &
+    FindRoomByPlayerPort &
+    RoomPhaseTransitionPort,
+  runtimeRegistry: FindGameByRoomPort & FindGameByPlayerPort,
 ) => {
   const common = createCommonHandlerContext(io, socket);
   const gameOutputAdapter = createGameOutputAdapter(common);
@@ -50,19 +53,19 @@
   const { guardOnEvent } = createPayloadGuard(socket.id);
   const guardPingPayload = guardOnEvent(
     protocol.SocketEvents.PING,
-    gamePayloadValidators[protocol.SocketEvents.PING]
+    gamePayloadValidators[protocol.SocketEvents.PING],
   );
   const guardMovePayload = guardOnEvent(
     protocol.SocketEvents.MOVE,
-    gamePayloadValidators[protocol.SocketEvents.MOVE]
+    gamePayloadValidators[protocol.SocketEvents.MOVE],
   );
   const guardPlaceBombPayload = guardOnEvent(
     protocol.SocketEvents.PLACE_BOMB,
-    gamePayloadValidators[protocol.SocketEvents.PLACE_BOMB]
+    gamePayloadValidators[protocol.SocketEvents.PLACE_BOMB],
   );
   const guardBombHitReportPayload = guardOnEvent(
     protocol.SocketEvents.BOMB_HIT_REPORT,
-    gamePayloadValidators[protocol.SocketEvents.BOMB_HIT_REPORT]
+    gamePayloadValidators[protocol.SocketEvents.BOMB_HIT_REPORT],
   );
   // 遅延計測用のPINGを検証しPONGを返す
   onEvent(protocol.SocketEvents.PING, (clientTime) => {
@@ -77,9 +80,14 @@
   });
 
   // オーナー開始要求に応じてゲーム進行ユースケースを起動する
-  onEvent(protocol.SocketEvents.START_GAME, () => {
+  onEvent(protocol.SocketEvents.START_GAME, (data) => {
+    if (!isStartGamePayload(data)) {
+      return;
+    }
+
     startGameCoordinator({
       ownerId: socket.id,
+      requestedPlayerCount: data.targetPlayerCount,
       roomManager,
       runtimeRegistry,
       output: gameOutputAdapter,
@@ -102,7 +110,11 @@
       return;
     }
 
-    const runtime = resolveRuntimeByPlayerId(roomManager, runtimeRegistry, socket.id);
+    const runtime = resolveRuntimeByPlayerId(
+      roomManager,
+      runtimeRegistry,
+      socket.id,
+    );
     if (!runtime) {
       return;
     }
@@ -120,7 +132,11 @@
       return;
     }
 
-    const runtime = resolveRuntimeByPlayerId(roomManager, runtimeRegistry, socket.id);
+    const runtime = resolveRuntimeByPlayerId(
+      roomManager,
+      runtimeRegistry,
+      socket.id,
+    );
     if (!runtime) {
       return;
     }
@@ -143,7 +159,11 @@
       return;
     }
 
-    const runtime = resolveRuntimeByPlayerId(roomManager, runtimeRegistry, socket.id);
+    const runtime = resolveRuntimeByPlayerId(
+      roomManager,
+      runtimeRegistry,
+      socket.id,
+    );
     if (!runtime) {
       return;
     }
diff --git a/apps/server/src/network/validation/socketPayloadValidators.ts b/apps/server/src/network/validation/socketPayloadValidators.ts
index 5449c57..7f05d71 100644
--- a/apps/server/src/network/validation/socketPayloadValidators.ts
+++ b/apps/server/src/network/validation/socketPayloadValidators.ts
@@ -2,7 +2,12 @@
  * socketPayloadValidators
  * ソケット受信ペイロードの型ガードを提供する
  */
-import type { playerTypes, roomTypes, PlaceBombPayload, BombHitReportPayload } from "@repo/shared";
+import type {
+  playerTypes,
+  roomTypes,
+  PlaceBombPayload,
+  BombHitReportPayload,
+} from "@repo/shared";
 import type { PingPayload } from "@repo/shared";
 import { isPlaceBombPayload as isValidPlaceBombPayload } from "@server/domains/game/entities/bomb/bombPayloadValidation";
 
@@ -20,7 +25,9 @@
 };
 
 /** MOVEイベントのペイロードが移動座標であるか判定する */
-export const isMovePayload = (value: unknown): value is playerTypes.MovePayload => {
+export const isMovePayload = (
+  value: unknown,
+): value is playerTypes.MovePayload => {
   if (typeof value !== "object" || value === null) {
     return false;
   }
@@ -30,12 +37,16 @@
 };
 
 /** PLACE_BOMBイベントのペイロードが爆弾設置要求であるか判定する */
-export const isPlaceBombPayload = (value: unknown): value is PlaceBombPayload => {
+export const isPlaceBombPayload = (
+  value: unknown,
+): value is PlaceBombPayload => {
   return isValidPlaceBombPayload(value);
 };
 
 /** BOMB_HIT_REPORTイベントのペイロードが被弾報告であるか判定する */
-export const isBombHitReportPayload = (value: unknown): value is BombHitReportPayload => {
+export const isBombHitReportPayload = (
+  value: unknown,
+): value is BombHitReportPayload => {
   if (typeof value !== "object" || value === null) {
     return false;
   }
@@ -44,12 +55,37 @@
   return isNonEmptyString(candidate.bombId);
 };
 
-/** JOIN_ROOMイベントのペイロードが参加情報であるか判定する */
-export const isJoinRoomPayload = (value: unknown): value is roomTypes.JoinRoomPayload => {
+/** START_GAMEイベントのペイロードが開始要求情報であるか判定する */
+export const isStartGamePayload = (
+  value: unknown,
+): value is { targetPlayerCount?: number } => {
   if (typeof value !== "object" || value === null) {
     return false;
   }
 
   const candidate = value as Record<string, unknown>;
-  return isNonEmptyString(candidate.roomId) && isNonEmptyString(candidate.playerName);
+  const targetPlayerCount = candidate.targetPlayerCount;
+  if (targetPlayerCount === undefined) {
+    return true;
+  }
+
+  return (
+    typeof targetPlayerCount === "number" &&
+    Number.isInteger(targetPlayerCount) &&
+    targetPlayerCount > 0
+  );
+};
+
+/** JOIN_ROOMイベントのペイロードが参加情報であるか判定する */
+export const isJoinRoomPayload = (
+  value: unknown,
+): value is roomTypes.JoinRoomPayload => {
+  if (typeof value !== "object" || value === null) {
+    return false;
+  }
+
+  const candidate = value as Record<string, unknown>;
+  return (
+    isNonEmptyString(candidate.roomId) && isNonEmptyString(candidate.playerName)
+  );
 };
diff --git a/packages/shared/src/protocol/eventPayloads.ts b/packages/shared/src/protocol/eventPayloads.ts
index 0da02b4..471bbf7 100644
--- a/packages/shared/src/protocol/eventPayloads.ts
+++ b/packages/shared/src/protocol/eventPayloads.ts
@@ -24,6 +24,7 @@
   NewPlayerPayload,
   RemovePlayerPayload,
   GameStartPayload,
+  StartGameRequestPayload,
   MovePayload,
   PlaceBombPayload,
   BombPlacedPayload,
diff --git a/packages/shared/src/protocol/maps/gameEventPayloadMap.ts b/packages/shared/src/protocol/maps/gameEventPayloadMap.ts
index cc53619..7cc6fc9 100644
--- a/packages/shared/src/protocol/maps/gameEventPayloadMap.ts
+++ b/packages/shared/src/protocol/maps/gameEventPayloadMap.ts
@@ -11,6 +11,7 @@
   CurrentPlayersPayload,
   GameResultPayload,
   GameStartPayload,
+  StartGameRequestPayload,
   MovePayload,
   NewPlayerPayload,
   PlaceBombPayload,
@@ -24,7 +25,7 @@
 
 /** ゲーム関連のクライアント送信イベントペイロード対応表 */
 export type GameClientToServerEventPayloadMap = {
-  [SocketEvents.START_GAME]: undefined;
+  [SocketEvents.START_GAME]: StartGameRequestPayload;
   [SocketEvents.READY_FOR_GAME]: undefined;
   [SocketEvents.MOVE]: MovePayload;
   [SocketEvents.PLACE_BOMB]: PlaceBombPayload;
diff --git a/packages/shared/src/protocol/payloads/gamePayloads.ts b/packages/shared/src/protocol/payloads/gamePayloads.ts
index 4ba4ff6..a8d5999 100644
--- a/packages/shared/src/protocol/payloads/gamePayloads.ts
+++ b/packages/shared/src/protocol/payloads/gamePayloads.ts
@@ -5,7 +5,10 @@
  */
 import type { PlayerPositionUpdate } from "../../domains/game/game.type";
 import type { CellUpdate } from "../../domains/gridMap/gridMap.type";
-import type { MovePayload as PlayerMovePayload, PlayerData } from "../../domains/player/player.type";
+import type {
+  MovePayload as PlayerMovePayload,
+  PlayerData,
+} from "../../domains/player/player.type";
 
 /** GAME_RESULT イベントで送受信するランキング1行 */
 export type GameResultRanking = {
@@ -53,6 +56,11 @@
 /** GAME_START イベントで送受信するゲーム開始情報 */
 export type GameStartPayload = { startTime: number };
 
+/** START_GAME イベントで受信するゲーム開始要求 */
+export type StartGameRequestPayload = {
+  targetPlayerCount?: number;
+};
+
 /** MOVE イベントで送受信する移動入力情報 */
 export type MovePayload = PlayerMovePayload;