diff --git a/src/lib/services/task_results.ts b/src/lib/services/task_results.ts
index 4bbac7fa4..cce4582d5 100644
--- a/src/lib/services/task_results.ts
+++ b/src/lib/services/task_results.ts
@@ -32,7 +32,7 @@ import { NOT_FOUND } from '$lib/constants/http-response-status-codes';
 const statusById = await getSubmissionStatusMapWithId();
 const statusByName = await getSubmissionStatusMapWithName();
 
-export async function getTaskResults(userId: string): Promise<TaskResults> {
+export async function getTaskResults(userId: string | undefined): Promise<TaskResults> {
   // 問題と特定のユーザの回答状況を使ってデータを結合
   // 計算量: 問題数をN、特定のユーザの解答数をMとすると、O(N + M)になるはず。
   const mergedTasksMap = await getMergedTasksMap();
@@ -127,14 +127,16 @@ async function transferAnswers(
 //       with_mapをtrueにすると、taskIdを使って各TaskResultにO(1)でアクセスできる。
 // Why : データ総量を抑えるため。
 export async function getTaskResultsOnlyResultExists(
-  userId: string,
+  userId: string | undefined,
   with_map: boolean = false,
 ): Promise<TaskResults | Map<string, TaskResult>> {
   const taskResultsMap: Map<string, TaskResult> = new Map();
 
   // TODO: answerの降順にしたい
   const tasks = await getTasks();
-  const answers = await answer_crud.getAnswers(userId);
+  // Skip the DB round-trip for anonymous users: getAnswers(undefined) drops the
+  // WHERE filter and full-scans taskAnswer.
+  const answers = userId !== undefined ? await answer_crud.getAnswers(userId) : new Map();
   const tasksHasAnswer = tasks.filter((task) => answers.has(task.task_id));
   const taskResultsWithAnswer = tasksHasAnswer.map((task: Task) => {
     const taskResult = createDefaultTaskResult(userId, task);
@@ -215,9 +217,11 @@ export async function getTaskResultsByTaskId(
  * @param userId - User ID for creating TaskResults
  * @returns Promise<TaskResults> - Array of TaskResult objects
  */
-async function createTaskResults(tasks: Tasks, userId: string): Promise<TaskResults> {
-  const answers = await answer_crud.getAnswers(userId);
+async function createTaskResults(tasks: Tasks, userId: string | undefined): Promise<TaskResults> {
   const isLoggedIn = userId !== undefined;
+  // Skip the DB round-trip for anonymous users: getAnswers(undefined) drops the
+  // WHERE filter and full-scans taskAnswer.
+  const answers = isLoggedIn ? await answer_crud.getAnswers(userId) : new Map();
 
   return tasks.map((task: Task) => {
     const answer = isLoggedIn ? answers.get(task.task_id) : null;
@@ -236,7 +240,7 @@ async function createTaskResults(tasks: Tasks, userId: string): Promise<TaskResu
  */
 function mergeTaskAndAnswer(
   task: Task,
-  userId: string,
+  userId: string | undefined,
   answer: TaskAnswer | null | undefined,
 ): TaskResult {
   const taskResult = createDefaultTaskResult(userId, task);
@@ -263,7 +267,7 @@ function mergeTaskAndAnswer(
   return taskResult;
 }
 
-export function createDefaultTaskResult(userId: string, task: Task): TaskResult {
+export function createDefaultTaskResult(userId: string | undefined, task: Task): TaskResult {
   const taskResult: TaskResult = {
     contest_id: task.contest_id,
     task_id: task.task_id,
@@ -312,7 +316,7 @@ export async function updateTaskResult(taskId: string, submissionStatus: string,
 
 export async function getTasksWithTagIds(
   tagIds_string: string,
-  userId: string,
+  userId: string | undefined,
 ): Promise<TaskResults> {
   const tagIds = tagIds_string.split(',');
   const taskIdByTagIds = await db.taskTag.groupBy({
diff --git a/src/lib/types/task.ts b/src/lib/types/task.ts
index 3f09ee387..d32dc6ea3 100644
--- a/src/lib/types/task.ts
+++ b/src/lib/types/task.ts
@@ -61,8 +61,14 @@ export type TaskGrades = TaskGrade[];
 
 export const taskGradeValues = Object.values(TaskGrade);
 
+/**
+ * A user's submission result for a single task, extending {@link Task} with status metadata.
+ *
+ * Used for both authenticated and anonymous (logged-out) views.
+ */
 export interface TaskResult extends Task {
-  user_id: string;
+  /** Owner of this result; `undefined` for anonymous (logged-out) results. */
+  user_id: string | undefined;
   status_name: string;
   status_id: string;
   submission_status_image_path: string;
diff --git a/src/test/lib/services/task_results.test.ts b/src/test/lib/services/task_results.test.ts
index 42d02a79a..23aca1c3d 100644
--- a/src/test/lib/services/task_results.test.ts
+++ b/src/test/lib/services/task_results.test.ts
@@ -12,8 +12,11 @@
 
 import { describe, test, expect, vi, beforeEach } from 'vitest';
 
-import { getTaskResults } from '$lib/services/task_results';
-import type { TaskResult, TaskResults } from '$lib/types/task';
+import { getTasks } from '$lib/services/tasks';
+import { getAnswers } from '$lib/services/answers';
+import { getTaskResults, getTaskResultsOnlyResultExists } from '$lib/services/task_results';
+
+import type { TaskResult, TaskResults, Tasks } from '$lib/types/task';
 
 import {
   MOCK_TASKS_DATA,
@@ -381,6 +384,27 @@ describe('getTaskResults', () => {
     });
   });
 
+  describe('when anonymous (userId is undefined)', () => {
+    beforeEach(async () => {
+      mockAnswersForTest = MOCK_ANSWERS_WITH_ANSWERS;
+      vi.mocked(getAnswers).mockClear();
+      taskResults = await getTaskResults(undefined);
+    });
+
+    test('does not call getAnswers (skips the full-scan DB round-trip)', () => {
+      expect(getAnswers).not.toHaveBeenCalled();
+    });
+
+    test('returns default (未挑戦) results for all tasks', () => {
+      expect(taskResults.length).toBeGreaterThan(0);
+      taskResults.forEach((taskResult: TaskResult) => {
+        expect(taskResult.is_ac).toBe(false);
+        expect(taskResult.status_name).toBe('ns');
+        expect(taskResult.submission_status_label_name).toBe('未挑戦');
+      });
+    });
+  });
+
   describe('when answers exist', () => {
     beforeEach(async () => {
       mockAnswersForTest = MOCK_ANSWERS_WITH_ANSWERS;
@@ -411,6 +435,33 @@ describe('getTaskResults', () => {
   });
 });
 
+describe('getTaskResultsOnlyResultExists', () => {
+  describe('when anonymous (userId is undefined)', () => {
+    beforeEach(() => {
+      mockAnswersForTest = MOCK_ANSWERS_WITH_ANSWERS;
+      vi.mocked(getAnswers).mockClear();
+      vi.mocked(getTasks).mockResolvedValue([
+        {
+          id: '1',
+          contest_id: 'abc101',
+          task_id: 'arc099_a',
+          contest_type: 'ABC',
+          task_table_index: 'C',
+          title: 'Minimization',
+          grade: 'Q3',
+        },
+      ] as unknown as Tasks);
+    });
+
+    test('does not call getAnswers and returns an empty array', async () => {
+      const taskResults = await getTaskResultsOnlyResultExists(undefined, false);
+
+      expect(getAnswers).not.toHaveBeenCalled();
+      expect(taskResults).toEqual([]);
+    });
+  });
+});
+
 describe('mergeTaskAndAnswer', () => {
   createMergedTaskResults(
     'when no answers exist',