diff --git a/src/handlers/predictionHandlers.ts b/src/handlers/predictionHandlers.ts
index c687437..033fed4 100644
--- a/src/handlers/predictionHandlers.ts
+++ b/src/handlers/predictionHandlers.ts
@@ -24,6 +24,7 @@ export const prediction = onRequest(async (req, res) => {
     if (tail === "summary" && req.method === "GET") {
       const gameId = String(req.query.gameId ?? "");
       const result = await getSummary(gameId);
+      res.set("Cache-Control", "public, max-age=5");
       res.status(200).json(result);
       return;
     }
diff --git a/src/lib/memCache.ts b/src/lib/memCache.ts
new file mode 100644
index 0000000..0c385c8
--- /dev/null
+++ b/src/lib/memCache.ts
@@ -0,0 +1,72 @@
+/**
+ * 인스턴스 단위 인메모리 캐시 + promise coalescing.
+ *
+ * 동일 key에 대해 TTL 내에는 캐싱된 값을 반환하고,
+ * TTL 만료 후 동시에 여러 요청이 들어와도 fetcher를 한 번만 실행한다.
+ */
+export class MemCache<T> {
+  private cache = new Map<string, { data: T; expiresAt: number }>();
+  private inflight = new Map<string, Promise<T>>();
+
+  constructor(
+    private readonly ttlMs: number,
+    private readonly maxSize?: number,
+  ) {}
+
+  get(key: string): T | null {
+    const e = this.cache.get(key);
+    if (!e) return null;
+    if (Date.now() > e.expiresAt) {
+      this.cache.delete(key);
+      return null;
+    }
+    return e.data;
+  }
+
+  set(key: string, data: T): void {
+    this.evict();
+    if (this.maxSize !== undefined && this.cache.size >= this.maxSize) {
+      const oldest = this.cache.keys().next().value;
+      if (oldest !== undefined) this.cache.delete(oldest);
+    }
+    this.cache.set(key, { data, expiresAt: Date.now() + this.ttlMs });
+  }
+
+  private evict(): void {
+    const now = Date.now();
+    for (const [k, e] of this.cache) {
+      if (now > e.expiresAt) this.cache.delete(k);
+    }
+  }
+
+  delete(key: string): void {
+    this.cache.delete(key);
+    this.inflight.delete(key);
+  }
+
+  /**
+   * 캐시 조회 → miss면 fetcher 실행 (동일 key 중복 호출 방지).
+   */
+  async getOrFetch(key: string, fetcher: () => Promise<T>): Promise<T> {
+    const cached = this.get(key);
+    if (cached) return cached;
+
+    const existing = this.inflight.get(key);
+    if (existing) return existing;
+
+    const promise = fetcher().then(
+      (data) => {
+        this.set(key, data);
+        this.inflight.delete(key);
+        return data;
+      },
+      (err) => {
+        this.inflight.delete(key);
+        throw err;
+      },
+    );
+
+    this.inflight.set(key, promise);
+    return promise;
+  }
+}
\ No newline at end of file
diff --git a/src/services/gameListService.ts b/src/services/gameListService.ts
index a5a5220..33f88f4 100644
--- a/src/services/gameListService.ts
+++ b/src/services/gameListService.ts
@@ -5,6 +5,7 @@ import {
   SERIES_CODES,
   type GameListResult,
 } from "../kbo/game-list.js";
+import { MemCache } from "../lib/memCache.js";
 
 // 게임센터(GetKboGameList)는 다른 KBO 데이터와 달리 Firestore 공유 캐시
 // (kboCacheRepository) 대신 인스턴스 단위 인메모리 캐시를 쓴다.
@@ -18,32 +19,7 @@ import {
 //
 // 트래픽이 커져 인스턴스 수 × 호출량이 문제가 되면 Firestore 공유 캐시로
 // 격상을 재검토 (TODO.md 참조).
-const MEM_TTL_MS = 10_000;
-const MEM_MAX = 100;
-
-interface MemEntry {
-  data: GameListResult;
-  expiresAt: number;
-}
-const memCache = new Map<string, MemEntry>();
-
-function memGet(key: string): GameListResult | null {
-  const e = memCache.get(key);
-  if (!e) return null;
-  if (Date.now() > e.expiresAt) {
-    memCache.delete(key);
-    return null;
-  }
-  return e.data;
-}
-
-function memSet(key: string, data: GameListResult): void {
-  if (memCache.size >= MEM_MAX) {
-    const oldest = memCache.keys().next().value;
-    if (oldest !== undefined) memCache.delete(oldest);
-  }
-  memCache.set(key, { data, expiresAt: Date.now() + MEM_TTL_MS });
-}
+const gameListCache = new MemCache<GameListResult>(10_000);
 
 export async function getGameList(
   date: string | Date,
@@ -60,14 +36,7 @@ export async function getGameList(
     : `${date.getFullYear()}${String(date.getMonth() + 1).padStart(2, "0")}${String(date.getDate()).padStart(2, "0")}`;
   const key = `${dateStr}|${resolvedSeries ?? ""}|${resolvedLeague ?? ""}`;
 
-  const hit = memGet(key);
-  if (hit) return hit;
-
-  const result = await fetchGameList({
-    date,
-    series: resolvedSeries,
-    league: resolvedLeague,
-  });
-  memSet(key, result);
-  return result;
+  return gameListCache.getOrFetch(key, () =>
+    fetchGameList({ date, series: resolvedSeries, league: resolvedLeague }),
+  );
 }
diff --git a/src/services/predictionService.ts b/src/services/predictionService.ts
index a2062cd..b75a9d0 100644
--- a/src/services/predictionService.ts
+++ b/src/services/predictionService.ts
@@ -9,6 +9,11 @@ import {
 } from "../repositories/voteRepository.js";
 import type { Game, VoteEntry } from "../types/panit.js";
 import { fromTimestamp, parseDateString, type DateString } from "../types/dateString.js";
+import { MemCache } from "../lib/memCache.js";
+
+type SummaryCounts = { homeCount: number; awayCount: number };
+
+const summaryCache = new MemCache<SummaryCounts>(5_000);
 
 function gameDate(game: Game): DateString {
   return fromTimestamp(game.time);
@@ -49,6 +54,7 @@ export async function createPrediction(
     side,
     team: body.selectedTeamCode,
   });
+  summaryCache.delete(body.gameId);
   return { ok: true };
 }
 
@@ -76,6 +82,7 @@ export async function updatePrediction(
     newSide,
     newTeam: body.selectedTeamCode,
   });
+  summaryCache.delete(body.gameId);
   return { ok: true, changed: true };
 }
 
@@ -102,5 +109,5 @@ export async function getSummary(
   gameId: string
 ): Promise<{ homeCount: number; awayCount: number }> {
   if (!gameId) throw new HttpError(400, "gameId required");
-  return getCounts(gameId);
+  return summaryCache.getOrFetch(gameId, () => getCounts(gameId));
 }