diff --git a/tests/GitignoreValidationTest.php b/tests/GitignoreValidationTest.php
new file mode 100644
index 00000000..8c3c6417
--- /dev/null
+++ b/tests/GitignoreValidationTest.php
@@ -0,0 +1,160 @@
+<?php
+/**
+ * Test Framework: PHPUnit
+ * Purpose: Validate critical .gitignore patterns are present and consistent.
+ */
+use PHPUnit\Framework\TestCase;
+
+final class GitignoreValidationTest extends TestCase
+{
+    private static $path = __DIR__ . '/../.gitignore';
+    private static $expected = [
+        // OS-specific
+        '.DS_Store',
+        'Thumbs.db',
+        'ehthumbs.db',
+        'Desktop.ini',
+        // Temp/log/backup
+        '*.log',
+        '*.tmp',
+        '*.swp',
+        '*.swo',
+        '*.bak',
+        '*.orig',
+        // Secrets
+        '.env',
+        '.env.*',
+        '*.key',
+        '*.pem',
+        // Build/cache
+        'dist/',
+        'build/',
+        'out/',
+        'tmp/',
+        'cache/',
+        '*.pyc',
+        '__pycache__/',
+        // IDE
+        '.vscode/',
+        '.history/',
+        '.idea/',
+        // Node
+        'node_modules/',
+        'npm-debug.log',
+        'yarn-error.log',
+        '*.lock',
+        // PHP
+        'vendor/',
+        'composer.phar',
+        '.phpunit.result.cache',
+        'phpunit.xml.dist',
+        '.php-cs-fixer.cache',
+        '.phpstan/',
+        'psalm.xml.dist',
+        'storage/',
+        'bootstrap/cache/',
+        'var/',
+        '*.cache',
+        '.env.backup',
+        '.env.local',
+        '.env.production',
+        '.env.testing',
+        'homestead.yaml',
+        'Homestead.yaml',
+        'Homestead.json',
+        // WordPress
+        'wp-content/uploads/',
+        'wp-content/cache/',
+        'wp-content/upgrade/',
+        'wp-content/debug.log',
+        // Coverage
+        'coverage/',
+        '*.lcov',
+        // Other binaries
+        '*.iml',
+        '*.class',
+        '*.o',
+        '*.out',
+        '*.so',
+        '*.dll',
+        '*.exe',
+        '*.pid',
+    ];
+
+    private function readGitignore(): array
+    {
+        $this->assertFileExists(self::$path, ".gitignore must exist at repo root.");
+        $content = file_get_contents(self::$path);
+        $this->assertNotFalse($content, "Failed to read .gitignore");
+        // Normalize line endings and trim trailing spaces
+        $lines = preg_split('/\R/u', $content);
+        return array_map(static function ($l) {
+            return rtrim($l, " \t");
+        }, $lines);
+    }
+
+    public function test_expected_patterns_are_present(): void
+    {
+        $lines = $this->readGitignore();
+        $set = array_flip($lines);
+        foreach (self::$expected as $pattern) {
+            $this->assertArrayHasKey($pattern, $set, "Missing required .gitignore entry: {$pattern}");
+        }
+    }
+
+    public function test_no_trailing_whitespace_on_rule_lines(): void
+    {
+        $content = file_get_contents(self::$path);
+        $this->assertNotFalse($content);
+        $bad = [];
+        $i = 0;
+        foreach (preg_split('/\R/u', $content) as $line) {
+            $i++;
+            if ($line !== '' && $line[0] !== '#') {
+                if (preg_match('/[ \t]+$/', $line)) {
+                    $bad[] = $i;
+                }
+            }
+        }
+        $this->assertCount(0, $bad, "Trailing whitespace detected on lines: " . implode(', ', $bad));
+    }
+
+    public function test_has_trailing_newline_at_eof(): void
+    {
+        $content = file_get_contents(self::$path);
+        $this->assertNotFalse($content);
+        $this->assertTrue(substr($content, -1) === "\n", "Expected .gitignore to end with a newline.");
+    }
+
+    public function test_no_duplicate_rules(): void
+    {
+        $lines = $this->readGitignore();
+        $counts = [];
+        foreach ($lines as $line) {
+            if ($line === '' || str_starts_with($line, '#')) continue;
+            $counts[$line] = ($counts[$line] ?? 0) + 1;
+        }
+        $dupes = array_keys(array_filter($counts, fn($c) => $c > 1));
+        $this->assertEmpty($dupes, "Duplicate .gitignore rules found: " . implode(', ', $dupes));
+    }
+
+    public function test_section_headers_exist(): void
+    {
+        $lines = $this->readGitignore();
+        $headers = [
+            '# OS依存ファイル',
+            '# 一時ファイル・ログ・バックアップ',
+            '# 環境変数・機密情報',
+            '# ビルド成果物・キャッシュ',
+            '# IDE / エディタ設定',
+            '# Node.js関連',
+            '# PHP関連設定',
+            '# カバレッジ関連',
+            '# その他',
+        ];
+        $set = array_flip($lines);
+        foreach ($headers as $h) {
+            $this->assertArrayHasKey($h, $set, "Missing section header: {$h}");
+        }
+    }
+}
\ No newline at end of file