diff --git a/apps/client/src/scenes/lobby/LobbyScene.tsx b/apps/client/src/scenes/lobby/LobbyScene.tsx
index f46abfe..759e6d1 100644
--- a/apps/client/src/scenes/lobby/LobbyScene.tsx
+++ b/apps/client/src/scenes/lobby/LobbyScene.tsx
@@ -2,11 +2,18 @@
 import { domain } from "@repo/shared";
 import type { FieldSizePreset, StartGameRequestPayload, TeamAssignmentMode } from "@repo/shared";
 import { config } from "@client/config";
+
+/** ホスト側で管理するゲーム設定 */
+export type LobbyGameSettings = {
+  targetPlayerCount: number;
+  fieldSizePreset: FieldSizePreset;
+  teamAssignmentMode: TeamAssignmentMode;
+};
 import { socketManager } from "@client/network/SocketManager";
 import { OVERLAY_BUTTON_STYLE } from "@client/scenes/shared/styles/overlayStyles";
 import { GearIcon } from "./components/GearIcon";
 import { LobbyRuleModal } from "./components/LobbyRuleModal";
-import { LobbySettingsModal } from "./components/LobbySettingsModal";
+import { LobbySettingsModal, toFieldPresetLabel } from "./components/LobbySettingsModal";
 import { LobbyStartConfirmModal } from "./components/LobbyStartConfirmModal";
 import {
   LOBBY_BACK_BUTTON_STYLE,
@@ -79,33 +86,22 @@
     return options;
   }, [minimumStartPlayerCount, maxStartPlayerCount]);
 
-  const [selectedStartPlayerCount, setSelectedStartPlayerCount] = useState(
-    minimumStartPlayerCount,
-  );
-  const fieldPresetOptions = useMemo(() => {
-    return Object.keys(
-      config.GAME_CONFIG.FIELD_PRESETS,
-    ) as FieldSizePreset[];
-  }, []);
-  const [selectedFieldSizePreset, setSelectedFieldSizePreset] =
-    useState<FieldSizePreset>(config.GAME_CONFIG.DEFAULT_FIELD_PRESET);
-  const [selectedTeamAssignmentMode, setSelectedTeamAssignmentMode] =
-    useState<TeamAssignmentMode>("random");
+  const [gameSettings, setGameSettings] = useState<LobbyGameSettings>({
+    targetPlayerCount: minimumStartPlayerCount,
+    fieldSizePreset: config.GAME_CONFIG.DEFAULT_FIELD_PRESET,
+    teamAssignmentMode: "random",
+  });
   const [teamFullMessage, setTeamFullMessage] = useState<string | null>(null);
   const [isRuleModalOpen, setIsRuleModalOpen] = useState(false);
   const [isStartConfirmVisible, setIsStartConfirmVisible] = useState(false);
   const [isSettingsModalOpen, setIsSettingsModalOpen] = useState(false);
 
   useEffect(() => {
-    setSelectedStartPlayerCount((prev) => {
-      if (prev < minimumStartPlayerCount || prev > maxStartPlayerCount) {
-        return minimumStartPlayerCount;
+    setGameSettings((prev) => {
+      const count = prev.targetPlayerCount;
+      if (count < minimumStartPlayerCount || count > maxStartPlayerCount || count % teamUnit !== 0) {
+        return { ...prev, targetPlayerCount: minimumStartPlayerCount };
       }
-
-      if (prev % teamUnit !== 0) {
-        return minimumStartPlayerCount;
-      }
-
       return prev;
     });
   }, [minimumStartPlayerCount, maxStartPlayerCount]);
@@ -117,11 +113,11 @@
     }
 
     socketManager.lobby.updateLobbySettings({
-      targetPlayerCount: selectedStartPlayerCount,
-      fieldSizePreset: selectedFieldSizePreset,
-      teamAssignmentMode: selectedTeamAssignmentMode,
+      targetPlayerCount: gameSettings.targetPlayerCount,
+      fieldSizePreset: gameSettings.fieldSizePreset,
+      teamAssignmentMode: gameSettings.teamAssignmentMode,
     });
-  }, [isMeOwner, selectedStartPlayerCount, selectedFieldSizePreset, selectedTeamAssignmentMode]);
+  }, [isMeOwner, gameSettings.targetPlayerCount, gameSettings.fieldSizePreset, gameSettings.teamAssignmentMode]);
 
   const handleStartClick = () => {
     setIsStartConfirmVisible(true);
@@ -130,8 +126,8 @@
   const handleStartConfirm = () => {
     setIsStartConfirmVisible(false);
     onStart({
-      targetPlayerCount: selectedStartPlayerCount,
-      fieldSizePreset: selectedFieldSizePreset,
+      targetPlayerCount: gameSettings.targetPlayerCount,
+      fieldSizePreset: gameSettings.fieldSizePreset,
     });
   };
 
@@ -152,20 +148,6 @@
     socketManager.lobby.selectTeam({ preferredTeamId });
   };
 
-  const toFieldPresetLabel = (preset: FieldSizePreset): string => {
-    const range = config.GAME_CONFIG.FIELD_PRESETS[preset].recommendedPlayers;
-    const baseLabel =
-      preset === "SMALL"
-        ? "小"
-        : preset === "MEDIUM"
-          ? "中"
-          : preset === "LARGE"
-            ? "大"
-            : "極大";
-
-    return `${baseLabel} (${range.min}-${range.max}人目安)`;
-  };
-
   // 自分のチーム選択状態を取得する
   const myPreferredTeamId = room.players.find((p) => p.id === myId)?.preferredTeamId ?? null;
 
@@ -357,14 +339,8 @@
       {isSettingsModalOpen && (
         <LobbySettingsModal
           startPlayerCountOptions={startPlayerCountOptions}
-          selectedStartPlayerCount={selectedStartPlayerCount}
-          onChangeStartPlayerCount={setSelectedStartPlayerCount}
-          fieldPresetOptions={fieldPresetOptions}
-          selectedFieldSizePreset={selectedFieldSizePreset}
-          onChangeFieldSizePreset={setSelectedFieldSizePreset}
-          toFieldPresetLabel={toFieldPresetLabel}
-          selectedTeamAssignmentMode={selectedTeamAssignmentMode}
-          onChangeTeamAssignmentMode={setSelectedTeamAssignmentMode}
+          settings={gameSettings}
+          onChangeSettings={setGameSettings}
           onClose={() => { setIsSettingsModalOpen(false); }}
         />
       )}
diff --git a/apps/client/src/scenes/lobby/components/LobbySettingsModal.tsx b/apps/client/src/scenes/lobby/components/LobbySettingsModal.tsx
index 8e63589..5fd2bcb 100644
--- a/apps/client/src/scenes/lobby/components/LobbySettingsModal.tsx
+++ b/apps/client/src/scenes/lobby/components/LobbySettingsModal.tsx
@@ -3,8 +3,9 @@
  * ホスト用のゲーム設定（人数・フィールドサイズ・チーム割り当て方式）を変更するモーダル
  */
 import type { FieldSizePreset } from "@repo/shared";
-import type { TeamAssignmentMode } from "@repo/shared";
+import { config } from "@client/config";
 import { OVERLAY_BUTTON_STYLE } from "@client/scenes/shared/styles/overlayStyles";
+import type { LobbyGameSettings } from "../LobbyScene";
 import {
   LOBBY_SETTINGS_MODAL_BODY_STYLE,
   LOBBY_SETTINGS_MODAL_FIELD_STYLE,
@@ -18,28 +19,31 @@
 
 type LobbySettingsModalProps = {
   startPlayerCountOptions: number[];
-  selectedStartPlayerCount: number;
-  onChangeStartPlayerCount: (count: number) => void;
-  fieldPresetOptions: FieldSizePreset[];
-  selectedFieldSizePreset: FieldSizePreset;
-  onChangeFieldSizePreset: (preset: FieldSizePreset) => void;
-  toFieldPresetLabel: (preset: FieldSizePreset) => string;
-  selectedTeamAssignmentMode: TeamAssignmentMode;
-  onChangeTeamAssignmentMode: (mode: TeamAssignmentMode) => void;
+  settings: LobbyGameSettings;
+  onChangeSettings: (settings: LobbyGameSettings) => void;
   onClose: () => void;
 };
 
+const FIELD_PRESET_OPTIONS = Object.keys(config.GAME_CONFIG.FIELD_PRESETS) as FieldSizePreset[];
+
+export const toFieldPresetLabel = (preset: FieldSizePreset): string => {
+  const range = config.GAME_CONFIG.FIELD_PRESETS[preset].recommendedPlayers;
+  const baseLabel =
+    preset === "SMALL"
+      ? "小"
+      : preset === "MEDIUM"
+        ? "中"
+        : preset === "LARGE"
+          ? "大"
+          : "極大";
+  return `${baseLabel} (${range.min}-${range.max}人目安)`;
+};
+
 /** ホスト用ゲーム設定ポップアップ */
 export const LobbySettingsModal = ({
   startPlayerCountOptions,
-  selectedStartPlayerCount,
-  onChangeStartPlayerCount,
-  fieldPresetOptions,
-  selectedFieldSizePreset,
-  onChangeFieldSizePreset,
-  toFieldPresetLabel,
-  selectedTeamAssignmentMode,
-  onChangeTeamAssignmentMode,
+  settings,
+  onChangeSettings,
   onClose,
 }: LobbySettingsModalProps) => {
   return (
@@ -57,9 +61,9 @@
             </label>
             <select
               id="settings-modal-start-player-count"
-              value={selectedStartPlayerCount}
+              value={settings.targetPlayerCount}
               onChange={(event) => {
-                onChangeStartPlayerCount(Number(event.target.value));
+                onChangeSettings({ ...settings, targetPlayerCount: Number(event.target.value) });
               }}
               style={LOBBY_SETTINGS_MODAL_SELECT_STYLE}
             >
@@ -80,13 +84,13 @@
             </label>
             <select
               id="settings-modal-field-size-preset"
-              value={selectedFieldSizePreset}
+              value={settings.fieldSizePreset}
               onChange={(event) => {
-                onChangeFieldSizePreset(event.target.value as FieldSizePreset);
+                onChangeSettings({ ...settings, fieldSizePreset: event.target.value as FieldSizePreset });
               }}
               style={LOBBY_SETTINGS_MODAL_SELECT_STYLE}
             >
-              {fieldPresetOptions.map((preset) => (
+              {FIELD_PRESET_OPTIONS.map((preset) => (
                 <option key={preset} value={preset}>
                   {toFieldPresetLabel(preset)}
                 </option>
@@ -103,9 +107,12 @@
             </label>
             <select
               id="settings-modal-team-assignment-mode"
-              value={selectedTeamAssignmentMode}
+              value={settings.teamAssignmentMode}
               onChange={(event) => {
-                onChangeTeamAssignmentMode(event.target.value as TeamAssignmentMode);
+                onChangeSettings({
+                  ...settings,
+                  teamAssignmentMode: event.target.value as LobbyGameSettings["teamAssignmentMode"],
+                });
               }}
               style={LOBBY_SETTINGS_MODAL_SELECT_STYLE}
             >
diff --git a/apps/server/src/application/coordinators/startGameCoordinator.ts b/apps/server/src/application/coordinators/startGameCoordinator.ts
index aa608ed..61f63d7 100644
--- a/apps/server/src/application/coordinators/startGameCoordinator.ts
+++ b/apps/server/src/application/coordinators/startGameCoordinator.ts
@@ -68,9 +68,6 @@
   }
 
   const updatedRoom = transitionResult.room;
-  if (!updatedRoom) {
-    return;
-  }
 
   const resolvedFieldSizePreset: FieldSizePreset =
     requestedFieldSizePreset
diff --git a/apps/server/src/domains/room/RoomManager.ts b/apps/server/src/domains/room/RoomManager.ts
index 10ec32f..73d1150 100644
--- a/apps/server/src/domains/room/RoomManager.ts
+++ b/apps/server/src/domains/room/RoomManager.ts
@@ -7,6 +7,8 @@
 import { RoomExitService } from "./application/services/RoomExitService";
 import { RoomPhaseService } from "./application/services/RoomPhaseService";
 import { RoomQueryService } from "./application/services/RoomQueryService";
+import { RoomSettingsService } from "./application/services/RoomSettingsService";
+import { RoomTeamService } from "./application/services/RoomTeamService";
 import type {
   JoinRoomResult,
   RoomDisconnectResult,
@@ -21,12 +23,16 @@
   private roomExitService: RoomExitService;
   private roomPhaseService: RoomPhaseService;
   private roomQueryService: RoomQueryService;
+  private roomSettingsService: RoomSettingsService;
+  private roomTeamService: RoomTeamService;
 
   constructor() {
     this.roomJoinService = new RoomJoinService(this.rooms);
     this.roomExitService = new RoomExitService(this.rooms);
     this.roomPhaseService = new RoomPhaseService(this.rooms);
     this.roomQueryService = new RoomQueryService(this.rooms);
+    this.roomSettingsService = new RoomSettingsService(this.rooms);
+    this.roomTeamService = new RoomTeamService(this.roomQueryService);
   }
 
   // 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
@@ -71,48 +77,15 @@
     fieldSizePreset: domain.room.Room["fieldSizePreset"],
     teamAssignmentMode: domain.room.TeamAssignmentMode,
   ): domain.room.Room | undefined {
-    const room = this.rooms.get(roomId);
-    if (!room || room.status !== domain.room.RoomPhase.WAITING) {
-      return undefined;
-    }
-
-    room.targetPlayerCount = targetPlayerCount;
-    room.fieldSizePreset = fieldSizePreset;
-    room.teamAssignmentMode = teamAssignmentMode;
-    return room;
+    return this.roomSettingsService.updateLobbySettings(roomId, targetPlayerCount, fieldSizePreset, teamAssignmentMode);
   }
 
-  // プレイヤーのチーム選択を更新して所属ルームを返す
   // プレイヤーのチーム選択を更新する，チームが満員なら team_full を返す
   public selectTeam(
     playerId: string,
     preferredTeamId: number | null,
   ): SelectTeamResult {
-    const room = this.roomQueryService.getRoomByPlayerId(playerId);
-    if (!room || room.status !== domain.room.RoomPhase.WAITING) {
-      return { status: "not_found" };
-    }
-
-    const member = room.players.find((p) => p.id === playerId);
-    if (!member) {
-      return { status: "not_found" };
-    }
-
-    // チーム指定がある場合は人数上限を確認する（上限 = ゲーム人数 / 4）
-    if (preferredTeamId !== null) {
-      const maxPerTeam = Math.floor(
-        (room.targetPlayerCount ?? room.maxPlayers) / 4,
-      );
-      const currentCount = room.players.filter(
-        (p) => p.preferredTeamId === preferredTeamId && p.id !== playerId,
-      ).length;
-      if (currentCount >= maxPerTeam) {
-        return { status: "team_full", teamId: preferredTeamId };
-      }
-    }
-
-    member.preferredTeamId = preferredTeamId;
-    return { status: "ok", room };
+    return this.roomTeamService.selectTeam(playerId, preferredTeamId);
   }
 
   // ルームを削除する
diff --git a/apps/server/src/domains/room/application/ports/roomUseCasePorts.ts b/apps/server/src/domains/room/application/ports/roomUseCasePorts.ts
index 4c73b59..4d0e627 100644
--- a/apps/server/src/domains/room/application/ports/roomUseCasePorts.ts
+++ b/apps/server/src/domains/room/application/ports/roomUseCasePorts.ts
@@ -71,10 +71,10 @@
 }
 
 /** ルーム状態遷移の実行結果 */
-export type RoomPhaseTransitionResult = {
-  status: "updated" | "not_found" | "invalid_transition";
-  room?: domain.room.Room;
-};
+export type RoomPhaseTransitionResult =
+  | { status: "updated"; room: domain.room.Room }
+  | { status: "not_found" }
+  | { status: "invalid_transition" };
 
 /** ルームIDでの存在確認に利用する参照ポート */
 export interface FindRoomByIdPort {
diff --git a/apps/server/src/domains/room/application/services/RoomPhaseService.ts b/apps/server/src/domains/room/application/services/RoomPhaseService.ts
index 3ee0466..eaea5f9 100644
--- a/apps/server/src/domains/room/application/services/RoomPhaseService.ts
+++ b/apps/server/src/domains/room/application/services/RoomPhaseService.ts
@@ -16,10 +16,7 @@
     }
 
     if (room.status === domain.room.RoomPhase.PLAYING) {
-      return {
-        status: "invalid_transition",
-        room,
-      };
+      return { status: "invalid_transition" };
     }
 
     room.status = domain.room.RoomPhase.PLAYING;
@@ -36,10 +33,7 @@
     }
 
     if (room.status === domain.room.RoomPhase.WAITING) {
-      return {
-        status: "invalid_transition",
-        room,
-      };
+      return { status: "invalid_transition" };
     }
 
     room.status = domain.room.RoomPhase.WAITING;
diff --git a/apps/server/src/domains/room/application/services/RoomSettingsService.ts b/apps/server/src/domains/room/application/services/RoomSettingsService.ts
new file mode 100644
index 0000000..2927f93
--- /dev/null
+++ b/apps/server/src/domains/room/application/services/RoomSettingsService.ts
@@ -0,0 +1,27 @@
+/**
+ * RoomSettingsService
+ * ロビー設定（ゲーム人数・フィールドサイズ・チーム割り当て方式）の更新処理を提供する
+ */
+import { domain } from "@repo/shared";
+
+/** ロビー設定の更新処理を担うサービス */
+export class RoomSettingsService {
+  constructor(private rooms: Map<string, domain.room.Room>) {}
+
+  public updateLobbySettings(
+    roomId: string,
+    targetPlayerCount: number,
+    fieldSizePreset: domain.room.Room["fieldSizePreset"],
+    teamAssignmentMode: domain.room.TeamAssignmentMode,
+  ): domain.room.Room | undefined {
+    const room = this.rooms.get(roomId);
+    if (!room || room.status !== domain.room.RoomPhase.WAITING) {
+      return undefined;
+    }
+
+    room.targetPlayerCount = targetPlayerCount;
+    room.fieldSizePreset = fieldSizePreset;
+    room.teamAssignmentMode = teamAssignmentMode;
+    return room;
+  }
+}
diff --git a/apps/server/src/domains/room/application/services/RoomTeamService.ts b/apps/server/src/domains/room/application/services/RoomTeamService.ts
new file mode 100644
index 0000000..db5c5e7
--- /dev/null
+++ b/apps/server/src/domains/room/application/services/RoomTeamService.ts
@@ -0,0 +1,45 @@
+/**
+ * RoomTeamService
+ * プレイヤーのチーム選択処理（上限チェックを含む）を提供する
+ */
+import { domain } from "@repo/shared";
+import type { SelectTeamResult } from "../ports/roomUseCasePorts";
+import type { RoomQueryService } from "./RoomQueryService";
+
+/** プレイヤーのチーム選択処理を担うサービス */
+export class RoomTeamService {
+  constructor(
+    private roomQueryService: RoomQueryService,
+  ) {}
+
+  public selectTeam(
+    playerId: string,
+    preferredTeamId: number | null,
+  ): SelectTeamResult {
+    const room = this.roomQueryService.getRoomByPlayerId(playerId);
+    if (!room || room.status !== domain.room.RoomPhase.WAITING) {
+      return { status: "not_found" };
+    }
+
+    const member = room.players.find((p) => p.id === playerId);
+    if (!member) {
+      return { status: "not_found" };
+    }
+
+    // チーム指定がある場合は人数上限を確認する（上限 = ゲーム人数 / 4）
+    if (preferredTeamId !== null) {
+      const maxPerTeam = Math.floor(
+        (room.targetPlayerCount ?? room.maxPlayers) / 4,
+      );
+      const currentCount = room.players.filter(
+        (p) => p.preferredTeamId === preferredTeamId && p.id !== playerId,
+      ).length;
+      if (currentCount >= maxPerTeam) {
+        return { status: "team_full", teamId: preferredTeamId };
+      }
+    }
+
+    member.preferredTeamId = preferredTeamId;
+    return { status: "ok", room };
+  }
+}
diff --git a/apps/server/src/network/handlers/room/roomEventOrchestrators.ts b/apps/server/src/network/handlers/room/roomEventOrchestrators.ts
index 35b97c6..2c50be4 100644
--- a/apps/server/src/network/handlers/room/roomEventOrchestrators.ts
+++ b/apps/server/src/network/handlers/room/roomEventOrchestrators.ts
@@ -53,6 +53,15 @@
     return;
   }
 
+  // 値に変化がなければ更新・配信をスキップする
+  if (
+    room.targetPlayerCount === payload.targetPlayerCount &&
+    room.fieldSizePreset === payload.fieldSizePreset &&
+    room.teamAssignmentMode === payload.teamAssignmentMode
+  ) {
+    return;
+  }
+
   const updatedRoom = deps.roomManager.updateLobbySettings(
     room.roomId,
     payload.targetPlayerCount,