diff --git a/src/__tests__/github-pr.test.ts b/src/__tests__/github-pr.test.ts
index 2e640d4..895dd76 100644
--- a/src/__tests__/github-pr.test.ts
+++ b/src/__tests__/github-pr.test.ts
@@ -1,14 +1,64 @@
 /**
  * Tests for github/pr module
  *
- * Tests buildPrBody formatting.
- * createPullRequest/pushBranch call `gh`/`git` CLI, not unit-tested here.
+ * Tests buildPrBody formatting and findExistingPr logic.
+ * createPullRequest/pushBranch/commentOnPr call `gh`/`git` CLI, not unit-tested here.
  */
 
-import { describe, it, expect } from 'vitest';
-import { buildPrBody } from '../infra/github/pr.js';
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+const mockExecFileSync = vi.fn();
+vi.mock('node:child_process', () => ({
+  execFileSync: (...args: unknown[]) => mockExecFileSync(...args),
+}));
+
+vi.mock('../infra/github/issue.js', () => ({
+  checkGhCli: vi.fn().mockReturnValue({ available: true }),
+}));
+
+vi.mock('../shared/utils/index.js', async (importOriginal) => ({
+  ...(await importOriginal<Record<string, unknown>>()),
+  createLogger: () => ({
+    info: vi.fn(),
+    debug: vi.fn(),
+    error: vi.fn(),
+  }),
+  getErrorMessage: (e: unknown) => String(e),
+}));
+
+import { buildPrBody, findExistingPr } from '../infra/github/pr.js';
 import type { GitHubIssue } from '../infra/github/types.js';
 
+describe('findExistingPr', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('オープンな PR がある場合はその PR を返す', () => {
+    mockExecFileSync.mockReturnValue(JSON.stringify([{ number: 42, url: 'https://github.com/org/repo/pull/42' }]));
+
+    const result = findExistingPr('/project', 'task/fix-bug');
+
+    expect(result).toEqual({ number: 42, url: 'https://github.com/org/repo/pull/42' });
+  });
+
+  it('PR がない場合は undefined を返す', () => {
+    mockExecFileSync.mockReturnValue(JSON.stringify([]));
+
+    const result = findExistingPr('/project', 'task/fix-bug');
+
+    expect(result).toBeUndefined();
+  });
+
+  it('gh CLI が失敗した場合は undefined を返す', () => {
+    mockExecFileSync.mockImplementation(() => { throw new Error('gh: command not found'); });
+
+    const result = findExistingPr('/project', 'task/fix-bug');
+
+    expect(result).toBeUndefined();
+  });
+});
+
 describe('buildPrBody', () => {
   it('should build body with single issue and report', () => {
     const issue: GitHubIssue = {
diff --git a/src/__tests__/postExecution.test.ts b/src/__tests__/postExecution.test.ts
new file mode 100644
index 0000000..d1d6adf
--- /dev/null
+++ b/src/__tests__/postExecution.test.ts
@@ -0,0 +1,116 @@
+/**
+ * Tests for postExecution.ts
+ *
+ * Verifies branching logic: existing PR → comment, no PR → create.
+ */
+
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+const { mockAutoCommitAndPush, mockPushBranch, mockFindExistingPr, mockCommentOnPr, mockCreatePullRequest, mockBuildPrBody } =
+  vi.hoisted(() => ({
+    mockAutoCommitAndPush: vi.fn(),
+    mockPushBranch: vi.fn(),
+    mockFindExistingPr: vi.fn(),
+    mockCommentOnPr: vi.fn(),
+    mockCreatePullRequest: vi.fn(),
+    mockBuildPrBody: vi.fn(() => 'pr-body'),
+  }));
+
+vi.mock('../infra/task/index.js', () => ({
+  autoCommitAndPush: (...args: unknown[]) => mockAutoCommitAndPush(...args),
+}));
+
+vi.mock('../infra/github/index.js', () => ({
+  pushBranch: (...args: unknown[]) => mockPushBranch(...args),
+  findExistingPr: (...args: unknown[]) => mockFindExistingPr(...args),
+  commentOnPr: (...args: unknown[]) => mockCommentOnPr(...args),
+  createPullRequest: (...args: unknown[]) => mockCreatePullRequest(...args),
+  buildPrBody: (...args: unknown[]) => mockBuildPrBody(...args),
+}));
+
+vi.mock('../infra/config/index.js', () => ({
+  resolvePieceConfigValue: vi.fn(),
+}));
+
+vi.mock('../shared/prompt/index.js', () => ({
+  confirm: vi.fn(),
+}));
+
+vi.mock('../shared/ui/index.js', () => ({
+  info: vi.fn(),
+  error: vi.fn(),
+  success: vi.fn(),
+}));
+
+vi.mock('../shared/utils/index.js', async (importOriginal) => ({
+  ...(await importOriginal<Record<string, unknown>>()),
+  createLogger: () => ({
+    info: vi.fn(),
+    debug: vi.fn(),
+    error: vi.fn(),
+  }),
+}));
+
+import { postExecutionFlow } from '../features/tasks/execute/postExecution.js';
+
+const baseOptions = {
+  execCwd: '/clone',
+  projectCwd: '/project',
+  task: 'Fix the bug',
+  branch: 'task/fix-the-bug',
+  baseBranch: 'main',
+  shouldCreatePr: true,
+  pieceIdentifier: 'default',
+};
+
+describe('postExecutionFlow', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockAutoCommitAndPush.mockReturnValue({ success: true, commitHash: 'abc123' });
+    mockPushBranch.mockReturnValue(undefined);
+    mockCommentOnPr.mockReturnValue({ success: true });
+    mockCreatePullRequest.mockReturnValue({ success: true, url: 'https://github.com/org/repo/pull/1' });
+  });
+
+  it('既存PRがない場合は createPullRequest を呼ぶ', async () => {
+    mockFindExistingPr.mockReturnValue(undefined);
+
+    await postExecutionFlow(baseOptions);
+
+    expect(mockCreatePullRequest).toHaveBeenCalledTimes(1);
+    expect(mockCommentOnPr).not.toHaveBeenCalled();
+  });
+
+  it('既存PRがある場合は commentOnPr を呼び createPullRequest は呼ばない', async () => {
+    mockFindExistingPr.mockReturnValue({ number: 42, url: 'https://github.com/org/repo/pull/42' });
+
+    await postExecutionFlow(baseOptions);
+
+    expect(mockCommentOnPr).toHaveBeenCalledWith('/project', 42, 'pr-body');
+    expect(mockCreatePullRequest).not.toHaveBeenCalled();
+  });
+
+  it('shouldCreatePr が false の場合は PR 関連処理をスキップする', async () => {
+    await postExecutionFlow({ ...baseOptions, shouldCreatePr: false });
+
+    expect(mockFindExistingPr).not.toHaveBeenCalled();
+    expect(mockCommentOnPr).not.toHaveBeenCalled();
+    expect(mockCreatePullRequest).not.toHaveBeenCalled();
+  });
+
+  it('commit がない場合は PR 関連処理をスキップする', async () => {
+    mockAutoCommitAndPush.mockReturnValue({ success: true, commitHash: undefined });
+
+    await postExecutionFlow(baseOptions);
+
+    expect(mockFindExistingPr).not.toHaveBeenCalled();
+    expect(mockCreatePullRequest).not.toHaveBeenCalled();
+  });
+
+  it('branch がない場合は PR 関連処理をスキップする', async () => {
+    await postExecutionFlow({ ...baseOptions, branch: undefined });
+
+    expect(mockFindExistingPr).not.toHaveBeenCalled();
+    expect(mockCreatePullRequest).not.toHaveBeenCalled();
+  });
+});
diff --git a/src/features/tasks/execute/postExecution.ts b/src/features/tasks/execute/postExecution.ts
index f20ae61..6df53cb 100644
--- a/src/features/tasks/execute/postExecution.ts
+++ b/src/features/tasks/execute/postExecution.ts
@@ -10,7 +10,7 @@ import { confirm } from '../../../shared/prompt/index.js';
 import { autoCommitAndPush } from '../../../infra/task/index.js';
 import { info, error, success } from '../../../shared/ui/index.js';
 import { createLogger } from '../../../shared/utils/index.js';
-import { createPullRequest, buildPrBody, pushBranch } from '../../../infra/github/index.js';
+import { createPullRequest, buildPrBody, pushBranch, findExistingPr, commentOnPr } from '../../../infra/github/index.js';
 import type { GitHubIssue } from '../../../infra/github/index.js';
 
 const log = createLogger('postExecution');
@@ -56,25 +56,37 @@ export async function postExecutionFlow(options: PostExecutionOptions): Promise<
   }
 
   if (commitResult.success && commitResult.commitHash && branch && shouldCreatePr) {
-    info('Creating pull request...');
     try {
       pushBranch(projectCwd, branch);
     } catch (pushError) {
       log.info('Branch push from project cwd failed (may already exist)', { error: pushError });
     }
     const report = pieceIdentifier ? `Piece \`${pieceIdentifier}\` completed successfully.` : 'Task completed successfully.';
-    const prBody = buildPrBody(issues, report);
-    const prResult = createPullRequest(projectCwd, {
-      branch,
-      title: task.length > 100 ? `${task.slice(0, 97)}...` : task,
-      body: prBody,
-      base: baseBranch,
-      repo,
-    });
-    if (prResult.success) {
-      success(`PR created: ${prResult.url}`);
+    const existingPr = findExistingPr(projectCwd, branch);
+    if (existingPr) {
+      // PRが既に存在する場合はコメントを追加（push済みなので新コミットはPRに自動反映）
+      const commentBody = buildPrBody(issues, report);
+      const commentResult = commentOnPr(projectCwd, existingPr.number, commentBody);
+      if (commentResult.success) {
+        success(`PR updated with comment: ${existingPr.url}`);
+      } else {
+        error(`PR comment failed: ${commentResult.error}`);
+      }
     } else {
-      error(`PR creation failed: ${prResult.error}`);
+      info('Creating pull request...');
+      const prBody = buildPrBody(issues, report);
+      const prResult = createPullRequest(projectCwd, {
+        branch,
+        title: task.length > 100 ? `${task.slice(0, 97)}...` : task,
+        body: prBody,
+        base: baseBranch,
+        repo,
+      });
+      if (prResult.success) {
+        success(`PR created: ${prResult.url}`);
+      } else {
+        error(`PR creation failed: ${prResult.error}`);
+      }
     }
   }
 }
diff --git a/src/infra/github/index.ts b/src/infra/github/index.ts
index b085622..21e59e2 100644
--- a/src/infra/github/index.ts
+++ b/src/infra/github/index.ts
@@ -14,4 +14,5 @@ export {
   createIssue,
 } from './issue.js';
 
-export { pushBranch, createPullRequest, buildPrBody } from './pr.js';
+export type { ExistingPr } from './pr.js';
+export { pushBranch, createPullRequest, buildPrBody, findExistingPr, commentOnPr } from './pr.js';
diff --git a/src/infra/github/pr.ts b/src/infra/github/pr.ts
index 3b366bd..08b1668 100644
--- a/src/infra/github/pr.ts
+++ b/src/infra/github/pr.ts
@@ -13,6 +13,54 @@ export type { CreatePrOptions, CreatePrResult };
 
 const log = createLogger('github-pr');
 
+export interface ExistingPr {
+  number: number;
+  url: string;
+}
+
+/**
+ * Find an open PR for the given branch.
+ * Returns undefined if no PR exists.
+ */
+export function findExistingPr(cwd: string, branch: string): ExistingPr | undefined {
+  const ghStatus = checkGhCli();
+  if (!ghStatus.available) return undefined;
+
+  try {
+    const output = execFileSync(
+      'gh', ['pr', 'list', '--head', branch, '--state', 'open', '--json', 'number,url', '--limit', '1'],
+      { cwd, encoding: 'utf-8', stdio: ['pipe', 'pipe', 'pipe'] },
+    );
+    const prs = JSON.parse(output) as ExistingPr[];
+    return prs[0];
+  } catch {
+    return undefined;
+  }
+}
+
+/**
+ * Add a comment to an existing PR.
+ */
+export function commentOnPr(cwd: string, prNumber: number, body: string): CreatePrResult {
+  const ghStatus = checkGhCli();
+  if (!ghStatus.available) {
+    return { success: false, error: ghStatus.error ?? 'gh CLI is not available' };
+  }
+
+  try {
+    execFileSync('gh', ['pr', 'comment', String(prNumber), '--body', body], {
+      cwd,
+      encoding: 'utf-8',
+      stdio: ['pipe', 'pipe', 'pipe'],
+    });
+    return { success: true };
+  } catch (err) {
+    const errorMessage = getErrorMessage(err);
+    log.error('PR comment failed', { error: errorMessage });
+    return { success: false, error: errorMessage };
+  }
+}
+
 /**
  * Push a branch to origin.
  * Throws on failure.