diff --git a/src/lib/__tests__/githubYearInReview.test.ts b/src/lib/__tests__/githubYearInReview.test.ts
index 456afee..8e9fbf2 100644
--- a/src/lib/__tests__/githubYearInReview.test.ts
+++ b/src/lib/__tests__/githubYearInReview.test.ts
@@ -1,8 +1,32 @@
-import { describe, expect, it } from "vitest";
+import { describe, expect, it, vi, beforeEach, afterEach } from "vitest";
 
 import { getMostActiveDayFromCalendar, getMostActiveHour } from "@/lib/yearInReviewUtils";
 import { fetchYearInReviewData } from "@/lib/githubYearInReview";
-import { GitHubApiError } from "@/lib/types";
+import { GitHubApiError, RateLimitError, UserNotFoundError } from "@/lib/types";
+
+// "server-only" を事前にモック
+vi.mock("server-only", () => ({}));
+
+export const mockFetch = vi.fn();
+
+beforeEach(() => {
+  vi.stubGlobal("fetch", mockFetch);
+  mockFetch.mockReset();
+});
+
+afterEach(() => {
+  vi.unstubAllGlobals();
+});
+
+export function jsonResponse(data: unknown, status = 200, headers: Record<string, string> = {}): Response {
+  return {
+    ok: status >= 200 && status < 300,
+    status,
+    headers: new Headers(headers),
+    json: () => Promise.resolve(data),
+    text: () => Promise.resolve(JSON.stringify(data)),
+  } as unknown as Response;
+}
 
 describe("githubYearInReview helpers", () => {
     it("returns the hour with the highest summed activity", () => {
@@ -26,7 +50,7 @@ describe("githubYearInReview helpers", () => {
     });
 });
 
-describe("fetchYearInReviewData", () => {
+describe("fetchYearInReviewData error paths", () => {
     it("throws GitHubApiError when token is not provided", async () => {
         await expect(fetchYearInReviewData("testuser", 2024)).rejects.toThrow(GitHubApiError);
 
@@ -38,4 +62,90 @@ describe("fetchYearInReviewData", () => {
             expect((error as GitHubApiError).status).toBe(401);
         }
     });
+
+    it("throws UserNotFoundError when statsResponse.user is null", async () => {
+        let callCount = 0;
+        mockFetch.mockImplementation((url: string | URL | Request) => {
+            const urlStr = typeof url === "string" ? url : url instanceof URL ? url.toString() : (url as Request).url;
+            if (urlStr.includes("/graphql")) {
+                callCount++;
+                if (callCount === 1) {
+                    return Promise.resolve(jsonResponse({ data: { user: null } })); // statsPromise
+                }
+                if (callCount === 2) {
+                    return Promise.resolve(jsonResponse({
+                        data: {
+                            user: {
+                                contributionsCollection: {
+                                    commitContributionsByRepository: []
+                                }
+                            }
+                        }
+                    })); // reposResponse
+                }
+            }
+            return Promise.resolve(jsonResponse([], 200));
+        });
+
+        await expect(fetchYearInReviewData("nonexistent", 2024, "fake-token")).rejects.toThrow(UserNotFoundError);
+    });
+
+    it("throws UserNotFoundError when reposResponse.user is null", async () => {
+        let callCount = 0;
+        mockFetch.mockImplementation((url: string | URL | Request) => {
+            const urlStr = typeof url === "string" ? url : url instanceof URL ? url.toString() : (url as Request).url;
+            if (urlStr.includes("/graphql")) {
+                callCount++;
+                if (callCount === 1) {
+                    return Promise.resolve(jsonResponse({ data: { user: { contributionsCollection: {} } } }));
+                }
+                if (callCount === 2) {
+                    return Promise.resolve(jsonResponse({ data: { user: null } }));
+                }
+            }
+            return Promise.resolve(jsonResponse([], 200));
+        });
+
+        await expect(fetchYearInReviewData("nonexistent", 2024, "fake-token")).rejects.toThrow(UserNotFoundError);
+    });
+
+    it("throws RateLimitError when API returns 403", async () => {
+        let callCount = 0;
+        mockFetch.mockImplementation((url: string | URL | Request) => {
+            const urlStr = typeof url === "string" ? url : url instanceof URL ? url.toString() : (url as Request).url;
+            if (urlStr.includes("/graphql")) {
+                callCount++;
+                // Let statsPromise succeed so it doesn't cause unhandled rejection
+                if (callCount === 1) {
+                    return Promise.resolve(jsonResponse({ data: { user: { contributionsCollection: {} } } }));
+                }
+                // Let reposResponse fail
+                if (callCount === 2) {
+                    return Promise.resolve(jsonResponse(null, 403, { "X-RateLimit-Reset": "1700000000" }));
+                }
+            }
+            return Promise.resolve(jsonResponse([], 200));
+        });
+
+        await expect(fetchYearInReviewData("testuser", 2024, "fake-token")).rejects.toThrow(RateLimitError);
+    });
+
+    it("throws GitHubApiError when API returns other errors", async () => {
+        let callCount = 0;
+        mockFetch.mockImplementation((url: string | URL | Request) => {
+            const urlStr = typeof url === "string" ? url : url instanceof URL ? url.toString() : (url as Request).url;
+            if (urlStr.includes("/graphql")) {
+                callCount++;
+                if (callCount === 1) {
+                    return Promise.resolve(jsonResponse({ data: { user: { contributionsCollection: {} } } }));
+                }
+                if (callCount === 2) {
+                    return Promise.resolve(jsonResponse(null, 500));
+                }
+            }
+            return Promise.resolve(jsonResponse([], 200));
+        });
+
+        await expect(fetchYearInReviewData("testuser", 2024, "fake-token")).rejects.toThrow(GitHubApiError);
+    });
 });