diff --git a/apps/client/src/scenes/lobby/LobbyScene.tsx b/apps/client/src/scenes/lobby/LobbyScene.tsx
index 237bf95..f8dea50 100644
--- a/apps/client/src/scenes/lobby/LobbyScene.tsx
+++ b/apps/client/src/scenes/lobby/LobbyScene.tsx
@@ -4,6 +4,7 @@
 import { config } from "@client/config";
 import { OVERLAY_BUTTON_STYLE } from "@client/scenes/shared/styles/overlayStyles";
 import { LobbyRuleModal } from "./components/LobbyRuleModal";
+import { LobbyStartConfirmModal } from "./components/LobbyStartConfirmModal";
 import {
   LOBBY_BACK_BUTTON_STYLE,
   LOBBY_BACKGROUND_STYLE,
@@ -72,6 +73,7 @@
   const [selectedFieldSizePreset, setSelectedFieldSizePreset] =
     useState<FieldSizePreset>(config.GAME_CONFIG.DEFAULT_FIELD_PRESET);
   const [isRuleModalOpen, setIsRuleModalOpen] = useState(false);
+  const [isStartConfirmVisible, setIsStartConfirmVisible] = useState(false);
 
   useEffect(() => {
     setSelectedStartPlayerCount((prev) => {
@@ -87,13 +89,22 @@
     });
   }, [minimumStartPlayerCount, maxStartPlayerCount]);
 
-  const handleStart = () => {
+  const handleStartClick = () => {
+    setIsStartConfirmVisible(true);
+  };
+
+  const handleStartConfirm = () => {
+    setIsStartConfirmVisible(false);
     onStart({
       targetPlayerCount: selectedStartPlayerCount,
       fieldSizePreset: selectedFieldSizePreset,
     });
   };
 
+  const handleStartCancel = () => {
+    setIsStartConfirmVisible(false);
+  };
+
   const toFieldPresetLabel = (preset: FieldSizePreset): string => {
     const range = config.GAME_CONFIG.FIELD_PRESETS[preset].recommendedPlayers;
     const baseLabel =
@@ -191,7 +202,7 @@
                     ))}
                   </select>
 
-                  <button onClick={handleStart} style={LOBBY_START_BUTTON_STYLE}>
+                  <button onClick={handleStartClick} style={LOBBY_START_BUTTON_STYLE}>
                     ゲームスタート
                   </button>
 
@@ -249,6 +260,13 @@
           onClose={() => { setIsRuleModalOpen(false); }}
         />
       )}
+
+      {isStartConfirmVisible && (
+        <LobbyStartConfirmModal
+          onConfirm={handleStartConfirm}
+          onCancel={handleStartCancel}
+        />
+      )}
     </>
   );
 };
diff --git a/apps/client/src/scenes/lobby/components/LobbyStartConfirmModal.styles.ts b/apps/client/src/scenes/lobby/components/LobbyStartConfirmModal.styles.ts
new file mode 100644
index 0000000..53a567b
--- /dev/null
+++ b/apps/client/src/scenes/lobby/components/LobbyStartConfirmModal.styles.ts
@@ -0,0 +1,65 @@
+/**
+ * LobbyStartConfirmModal.styles
+ * ゲームスタート確認モーダルのスタイル定数を定義する
+ */
+import type { CSSProperties } from "react";
+import {
+  OVERLAY_PANEL_BASE_STYLE,
+  OVERLAY_PANEL_FOOTER_BASE_STYLE,
+  OVERLAY_PANEL_HEADER_BASE_STYLE,
+} from "@client/scenes/shared/styles/overlayStyles";
+
+/** オーバーレイ背景のスタイル */
+export const LOBBY_START_CONFIRM_OVERLAY_STYLE: CSSProperties = {
+  position: "fixed",
+  inset: 0,
+  background: "rgba(0, 0, 0, 0.72)",
+  zIndex: 120,
+  display: "flex",
+  alignItems: "center",
+  justifyContent: "center",
+  padding: "16px",
+};
+
+/** 確認パネルのスタイル */
+export const LOBBY_START_CONFIRM_PANEL_STYLE: CSSProperties = {
+  ...OVERLAY_PANEL_BASE_STYLE,
+  width: "min(360px, 100%)",
+  display: "flex",
+  flexDirection: "column",
+};
+
+/** 確認パネルヘッダーのスタイル */
+export const LOBBY_START_CONFIRM_HEADER_STYLE: CSSProperties = {
+  ...OVERLAY_PANEL_HEADER_BASE_STYLE,
+  fontSize: "1.1rem",
+  fontWeight: 800,
+};
+
+/** 確認パネル本文のスタイル */
+export const LOBBY_START_CONFIRM_BODY_STYLE: CSSProperties = {
+  padding: "20px 18px",
+  fontSize: "1rem",
+  lineHeight: 1.6,
+  color: "rgba(255, 255, 255, 0.85)",
+};
+
+/** 確認パネルフッターのスタイル */
+export const LOBBY_START_CONFIRM_FOOTER_STYLE: CSSProperties = {
+  ...OVERLAY_PANEL_FOOTER_BASE_STYLE,
+  display: "flex",
+  gap: "10px",
+  justifyContent: "flex-end",
+};
+
+/** 「はい」ボタンのスタイル */
+export const LOBBY_START_CONFIRM_YES_BUTTON_STYLE: CSSProperties = {
+  padding: "10px 24px",
+  fontSize: "0.95rem",
+  cursor: "pointer",
+  borderRadius: "8px",
+  border: "none",
+  background: "#4ade80",
+  color: "#111",
+  fontWeight: 700,
+};
diff --git a/apps/client/src/scenes/lobby/components/LobbyStartConfirmModal.tsx b/apps/client/src/scenes/lobby/components/LobbyStartConfirmModal.tsx
new file mode 100644
index 0000000..131e4a9
--- /dev/null
+++ b/apps/client/src/scenes/lobby/components/LobbyStartConfirmModal.tsx
@@ -0,0 +1,45 @@
+/**
+ * LobbyStartConfirmModal
+ * ゲームスタート前の確認ダイアログ
+ */
+import { OVERLAY_BUTTON_STYLE } from "@client/scenes/shared/styles/overlayStyles";
+import {
+  LOBBY_START_CONFIRM_BODY_STYLE,
+  LOBBY_START_CONFIRM_FOOTER_STYLE,
+  LOBBY_START_CONFIRM_HEADER_STYLE,
+  LOBBY_START_CONFIRM_OVERLAY_STYLE,
+  LOBBY_START_CONFIRM_PANEL_STYLE,
+  LOBBY_START_CONFIRM_YES_BUTTON_STYLE,
+} from "./LobbyStartConfirmModal.styles";
+
+type LobbyStartConfirmModalProps = {
+  onConfirm: () => void;
+  onCancel: () => void;
+};
+
+/** ゲームスタートの最終確認を表示するモーダル */
+export const LobbyStartConfirmModal = ({
+  onConfirm,
+  onCancel,
+}: LobbyStartConfirmModalProps) => {
+  return (
+    <div style={LOBBY_START_CONFIRM_OVERLAY_STYLE}>
+      <div style={LOBBY_START_CONFIRM_PANEL_STYLE}>
+        <div style={LOBBY_START_CONFIRM_HEADER_STYLE}>ゲームスタート</div>
+
+        <div style={LOBBY_START_CONFIRM_BODY_STYLE}>
+          ゲームを開始しますか？
+        </div>
+
+        <div style={LOBBY_START_CONFIRM_FOOTER_STYLE}>
+          <button onClick={onCancel} style={OVERLAY_BUTTON_STYLE}>
+            いいえ
+          </button>
+          <button onClick={onConfirm} style={LOBBY_START_CONFIRM_YES_BUTTON_STYLE}>
+            はい
+          </button>
+        </div>
+      </div>
+    </div>
+  );
+};
diff --git a/apps/client/src/scenes/lobby/styles/LobbyScene.styles.ts b/apps/client/src/scenes/lobby/styles/LobbyScene.styles.ts
index b93d465..768171e 100644
--- a/apps/client/src/scenes/lobby/styles/LobbyScene.styles.ts
+++ b/apps/client/src/scenes/lobby/styles/LobbyScene.styles.ts
@@ -112,8 +112,10 @@
   borderRadius: "8px",
   fontWeight: "bold",
   boxShadow: "0 4px 6px rgba(0,0,0,0.3)",
+  marginTop: "clamp(8px, 2dvh, 16px)",
 };
 
+
 /** 非オーナー待機表示のスタイル */
 export const LOBBY_WAITING_STYLE: CSSProperties = {
   padding: "20px",