diff --git a/apps/client/src/scenes/game/application/network/handlers/GameNetworkStateApplier.ts b/apps/client/src/scenes/game/application/network/handlers/GameNetworkStateApplier.ts
index 1df634d..f38e05c 100644
--- a/apps/client/src/scenes/game/application/network/handlers/GameNetworkStateApplier.ts
+++ b/apps/client/src/scenes/game/application/network/handlers/GameNetworkStateApplier.ts
@@ -9,6 +9,7 @@
   BombPlacedPayload,
   PlayerDeadPayload,
 } from "@repo/shared";
+import { domain } from "@repo/shared";
 import { AppearanceResolver } from "@client/scenes/game/application/AppearanceResolver";
 import { GameMapController } from "@client/scenes/game/entities/map/GameMapController";
 import { PlayerRepository } from "@client/scenes/game/entities/player/PlayerRepository";
@@ -105,7 +106,8 @@
         this.playerSyncHandler.handleRemovePlayer(payload);
       },
       onReceivedUpdateMapCells: (payload) => {
-        this.mapSyncHandler.handleUpdateMapCells(payload);
+        const updates = domain.game.gridMap.ungroupCellUpdates(payload);
+        this.mapSyncHandler.handleUpdateMapCells(updates);
       },
       onReceivedGameEnd: () => {
         this.onGameEnded();
diff --git a/apps/client/src/scenes/game/application/network/handlers/MapSyncHandler.ts b/apps/client/src/scenes/game/application/network/handlers/MapSyncHandler.ts
index 3762863..c2dd86b 100644
--- a/apps/client/src/scenes/game/application/network/handlers/MapSyncHandler.ts
+++ b/apps/client/src/scenes/game/application/network/handlers/MapSyncHandler.ts
@@ -1,10 +1,9 @@
 /**
  * MapSyncHandler
  * マップ同期イベントの受信処理を担当する
- * グループ化セル更新データを展開してマップへ適用する
+ * セル更新データをマップへ適用する
  */
-import type { UpdateMapCellsPayload } from "@repo/shared";
-import { domain } from "@repo/shared";
+import type { domain } from "@repo/shared";
 import { GameMapController } from "@client/scenes/game/entities/map/GameMapController";
 
 /** MapSyncHandler の初期化入力 */
@@ -20,9 +19,8 @@
     this.gameMap = gameMap;
   }
 
-  /** グループ化マップセル更新を展開して適用する */
-  public handleUpdateMapCells = (grouped: UpdateMapCellsPayload): void => {
-    const updates = domain.game.gridMap.ungroupCellUpdates(grouped);
+  /** マップセル更新を適用する */
+  public handleUpdateMapCells = (updates: domain.game.gridMap.CellUpdate[]): void => {
     this.gameMap.updateCells(updates);
   };
 }
\ No newline at end of file
diff --git a/apps/server/src/domains/game/application/ports/gameUseCasePorts.ts b/apps/server/src/domains/game/application/ports/gameUseCasePorts.ts
index 2c08386..c071fea 100644
--- a/apps/server/src/domains/game/application/ports/gameUseCasePorts.ts
+++ b/apps/server/src/domains/game/application/ports/gameUseCasePorts.ts
@@ -14,7 +14,6 @@
   GameStartPayload,
   PongPayload,
   RemovePlayerPayload,
-  UpdateMapCellsPayload,
   UpdatePlayersPayload,
 } from "@repo/shared";
 import type { GameSessionCallbacks } from "../services/GameRoomSession";
@@ -55,7 +54,7 @@
   ): void;
   publishMapCellUpdatesToRoom(
     roomId: domain.room.Room["roomId"],
-    cellUpdates: UpdateMapCellsPayload,
+    cellUpdates: domain.game.gridMap.CellUpdate[],
   ): void;
   publishGameEndToRoom(roomId: domain.room.Room["roomId"]): void;
   publishGameResultToRoom(
diff --git a/apps/server/src/domains/game/application/useCases/startGameUseCase.ts b/apps/server/src/domains/game/application/useCases/startGameUseCase.ts
index 96de2f4..c800c3c 100644
--- a/apps/server/src/domains/game/application/useCases/startGameUseCase.ts
+++ b/apps/server/src/domains/game/application/useCases/startGameUseCase.ts
@@ -14,7 +14,6 @@
   logScopes,
 } from "@server/logging/index";
 import { createBotBombActionHandler } from "../services/bot/index.js";
-import { domain } from "@repo/shared";
 
 const excludeRecipientFromPlayerUpdates = <
   TPlayerUpdate extends { id: string },
@@ -84,10 +83,7 @@
         }
 
         if (tickData.cellUpdates.length > 0) {
-          output.publishMapCellUpdatesToRoom(
-            roomId,
-            domain.game.gridMap.groupCellUpdates(tickData.cellUpdates),
-          );
+          output.publishMapCellUpdatesToRoom(roomId, tickData.cellUpdates);
         }
       },
       onGameEnd: (resultPayload) => {
diff --git a/apps/server/src/domains/game/entities/map/MapStore.ts b/apps/server/src/domains/game/entities/map/MapStore.ts
index bf09972..d02fffd 100644
--- a/apps/server/src/domains/game/entities/map/MapStore.ts
+++ b/apps/server/src/domains/game/entities/map/MapStore.ts
@@ -5,14 +5,14 @@
 import { domain } from "@repo/shared";
 import { createInitialGridColors } from "./mapGrid.js";
 import { paintCellIfChanged } from "./mapPainting.js";
-import { drainPendingUpdates } from "./mapUpdates.js";
+import { swapPendingUpdates } from "./mapUpdates.js";
 
 /** ルーム内マップの塗り状態と更新差分を管理するストア */
 export class MapStore {
   // 全マスの現在の色（teamId）を保持
   private gridColors: number[];
   // 次回の送信ループで送る差分リスト
-  private pendingUpdates: domain.game.gridMap.CellUpdate[];
+  public pendingUpdates: domain.game.gridMap.CellUpdate[];
 
   constructor() {
     // 初期状態は -1 (無色) などで初期化
@@ -36,7 +36,7 @@
     * 溜まっている差分を取得し，キューをクリアする（ループ送信時に使用）
    */
   public getAndClearUpdates(): domain.game.gridMap.CellUpdate[] {
-    return drainPendingUpdates(this.pendingUpdates);
+    return swapPendingUpdates(this);
   }
 
   /** 現在のマップ塗り状態をスナップショットとして返す */
diff --git a/apps/server/src/domains/game/entities/map/mapContestResolver.ts b/apps/server/src/domains/game/entities/map/mapContestResolver.ts
new file mode 100644
index 0000000..bc77662
--- /dev/null
+++ b/apps/server/src/domains/game/entities/map/mapContestResolver.ts
@@ -0,0 +1,47 @@
+/**
+ * mapContestResolver
+ * 同一セルに複数チームが重なった場合の競合判定を提供する
+ * 純粋関数として切り出し，単体テストを容易にする
+ */
+
+/** 同一セルに異なるチームが存在していることを示すセンチネル値 */
+export const CONTESTED_CELL = -2;
+
+/** プレイヤーごとのグリッド位置情報 */
+export type PlayerGridEntry = {
+  playerId: string;
+  gridIndex: number | null;
+  teamId: number;
+};
+
+/**
+ * 各セルの占有チームを判定し，競合セルは CONTESTED_CELL でマークする
+ * @returns セルインデックス → チームID（競合時は CONTESTED_CELL）のマップ
+ */
+export const resolveUncontestedCells = (
+  entries: PlayerGridEntry[],
+): Map<number, number> => {
+  const cellTeamMap = new Map<number, number>();
+
+  for (const { gridIndex, teamId } of entries) {
+    if (gridIndex === null) continue;
+
+    const existing = cellTeamMap.get(gridIndex);
+    if (existing === undefined) {
+      cellTeamMap.set(gridIndex, teamId);
+    } else if (existing !== CONTESTED_CELL && existing !== teamId) {
+      cellTeamMap.set(gridIndex, CONTESTED_CELL);
+    }
+  }
+
+  return cellTeamMap;
+};
+
+/** 指定セルが塗り可能（競合でない）かを判定する */
+export const isCellPaintable = (
+  cellTeamMap: Map<number, number>,
+  gridIndex: number,
+): boolean => {
+  const ownerTeamId = cellTeamMap.get(gridIndex);
+  return ownerTeamId !== undefined && ownerTeamId !== CONTESTED_CELL;
+};
diff --git a/apps/server/src/domains/game/entities/map/mapUpdates.ts b/apps/server/src/domains/game/entities/map/mapUpdates.ts
index 77ce63a..4446d55 100644
--- a/apps/server/src/domains/game/entities/map/mapUpdates.ts
+++ b/apps/server/src/domains/game/entities/map/mapUpdates.ts
@@ -4,11 +4,14 @@
  */
 import { domain } from "@repo/shared";
 
-/** 差分キューを配列として返却し，キューを空にする */
-export const drainPendingUpdates = (
-  pendingUpdates: domain.game.gridMap.CellUpdate[]
+/**
+ * 差分キューの参照をそのまま返却し，呼び出し元の配列を新しい空配列で差し替える
+ * スプレッドコピーを避けてゼロコピーで返却する
+ */
+export const swapPendingUpdates = (
+  owner: { pendingUpdates: domain.game.gridMap.CellUpdate[] },
 ): domain.game.gridMap.CellUpdate[] => {
-  const updates = [...pendingUpdates];
-  pendingUpdates.length = 0;
+  const updates = owner.pendingUpdates;
+  owner.pendingUpdates = [];
   return updates;
 };
diff --git a/apps/server/src/domains/game/loop/GameLoop.ts b/apps/server/src/domains/game/loop/GameLoop.ts
index d3f3d7b..bb3c464 100644
--- a/apps/server/src/domains/game/loop/GameLoop.ts
+++ b/apps/server/src/domains/game/loop/GameLoop.ts
@@ -5,6 +5,11 @@
 import { Player } from "../entities/player/Player.js";
 import { MapStore } from "../entities/map/MapStore";
 import { getPlayerGridIndex } from "../entities/player/playerPosition.js";
+import {
+  resolveUncontestedCells,
+  isCellPaintable,
+  type PlayerGridEntry,
+} from "../entities/map/mapContestResolver.js";
 import { config } from "@server/config";
 import { domain } from "@repo/shared";
 import type { PlaceBombPayload } from "@repo/shared";
@@ -246,32 +251,26 @@
   ): domain.game.tick.TickData["playerUpdates"] {
     const changedPlayers: domain.game.tick.TickData["playerUpdates"] = [];
 
-    // パス1: 各セルにどのチームが存在するか集計する
-    // 値が -1 のセルは複数チームが競合していることを示す
-    const cellTeamMap = new Map<number, number>();
-    const CONTESTED = -1;
-
+    // 全プレイヤーのグリッド位置を1度だけ計算してキャッシュする
+    const gridEntries: (PlayerGridEntry & { player: Player })[] = [];
     this.players.forEach((player) => {
-      const gridIndex = getPlayerGridIndex(player);
-      if (gridIndex === null) return;
-
-      const existing = cellTeamMap.get(gridIndex);
-      if (existing === undefined) {
-        cellTeamMap.set(gridIndex, player.teamId);
-      } else if (existing !== CONTESTED && existing !== player.teamId) {
-        cellTeamMap.set(gridIndex, CONTESTED);
-      }
+      gridEntries.push({
+        playerId: player.id,
+        gridIndex: getPlayerGridIndex(player),
+        teamId: player.teamId,
+        player,
+      });
     });
 
-    // パス2: 競合のないセルのみ塗り，プレイヤー差分を収集する
-    this.players.forEach((player) => {
-      activePlayerIds.add(player.id);
-      const gridIndex = getPlayerGridIndex(player);
-      if (gridIndex !== null) {
-        const ownerTeamId = cellTeamMap.get(gridIndex);
-        if (ownerTeamId !== undefined && ownerTeamId !== CONTESTED) {
-          this.mapStore.paintCell(gridIndex, player.teamId);
-        }
+    // 競合判定: 同一セルに異なるチームがいるかを解決する
+    const cellTeamMap = resolveUncontestedCells(gridEntries);
+
+    // 競合のないセルのみ塗り，プレイヤー差分を収集する
+    for (const { playerId, gridIndex, player } of gridEntries) {
+      activePlayerIds.add(playerId);
+
+      if (gridIndex !== null && isCellPaintable(cellTeamMap, gridIndex)) {
+        this.mapStore.paintCell(gridIndex, player.teamId);
       }
 
       // 送信用のプレイヤーデータを構築
@@ -291,7 +290,7 @@
         changedPlayers.push(playerData);
         this.lastSentPlayers.set(player.id, playerData);
       }
-    });
+    }
 
     return changedPlayers;
   }
diff --git a/apps/server/src/network/handlers/game/createGameOutputAdapter.ts b/apps/server/src/network/handlers/game/createGameOutputAdapter.ts
index f897f63..c8f85b3 100644
--- a/apps/server/src/network/handlers/game/createGameOutputAdapter.ts
+++ b/apps/server/src/network/handlers/game/createGameOutputAdapter.ts
@@ -3,7 +3,7 @@
  * ゲーム系ユースケースから利用する送信関数群を生成する
  */
 import { Server } from "socket.io";
-import { contracts as protocol } from "@repo/shared";
+import { contracts as protocol, domain as domainNs } from "@repo/shared";
 import type {
   BombPlacedAckPayload,
   BombPlacedPayload,
@@ -14,7 +14,6 @@
   PongPayload,
   CurrentPlayersPayload,
   RemovePlayerPayload,
-  UpdateMapCellsPayload,
   UpdatePlayersPayload,
 } from "@repo/shared";
 import type {
@@ -64,12 +63,13 @@
     },
     publishMapCellUpdatesToRoom: (
       roomId: RoomId,
-      cellUpdates: UpdateMapCellsPayload,
+      cellUpdates: domainNs.game.gridMap.CellUpdate[],
     ) => {
+      const grouped = domainNs.game.gridMap.groupCellUpdates(cellUpdates);
       common.emitToRoom(
         roomId,
         protocol.SocketEvents.UPDATE_MAP_CELLS,
-        cellUpdates,
+        grouped,
       );
     },
     publishGameEndToRoom: (roomId: RoomId) => {