diff --git a/bin/dependency-visualizer b/bin/dependency-visualizer
index 2a3f79f..5e301e6 100755
--- a/bin/dependency-visualizer
+++ b/bin/dependency-visualizer
@@ -7,13 +7,10 @@ use Hirokinoue\DependencyVisualizer\Config\Config;
 use Hirokinoue\DependencyVisualizer\Exporter\Exporter;
 use Hirokinoue\DependencyVisualizer\Exporter\PlantUmlExporter;
 use Hirokinoue\DependencyVisualizer\Exporter\StringExporter;
-use Monolog\Formatter\LineFormatter;
-use Monolog\Handler\NullHandler;
-use Monolog\Handler\StreamHandler;
-use Monolog\Logger;
 
 foreach ([__DIR__ . '/../../../autoload.php', __DIR__ . '/../vendor/autoload.php'] as $autoload) {
     if (file_exists($autoload)) {
+        $loadedFile = $autoload;
         require $autoload;
         break;
     }
@@ -35,30 +32,26 @@ Config::initialize(\getcwd());
 
 ini_set('memory_limit', Config::memoryLimit());
 
-$logger = new Logger('dependency visualizer');
-if ($options['log']) {
-    $handler = new StreamHandler(getcwd() . '/app.log');
-    $handler->setFormatter(new LineFormatter(null, null, true));
-    $logger->pushHandler($handler);
-} else {
-    $logger->pushHandler(new NullHandler());
-}
+Logger::initialize($options['log']);
 
-$logger->info('start analyzing', ['target' => $file]);
+Logger::info('autoload file has been loaded', ['path' => $loadedFile ?? '']);
+Logger::info('start analyzing', ['target' => $file]);
 
 try {
     $dependencyVisualizer = DependencyVisualizer::create($file);
     $diagramUnit = $dependencyVisualizer->analyze();
 } catch (\Exception $e) {
-    $logger->error($e->getMessage());
-    $logger->error('finish analyzing abnormally');
+    Logger::error($e->getMessage());
+    Logger::error('finish analyzing abnormally');
     exit(1);
 }
 
+Logger::info('start drawing');
+
 $exporter = exporterFactory($options);
 echo $exporter->export($diagramUnit);
 
-$logger->info('finish analyzing');
+Logger::info('finish analyzing');
 exit(0);
 
 function exporterFactory($options): Exporter {
diff --git a/src/Logger.php b/src/Logger.php
new file mode 100644
index 0000000..34d9e94
--- /dev/null
+++ b/src/Logger.php
@@ -0,0 +1,47 @@
+<?php declare(strict_types=1);
+
+namespace Hirokinoue\DependencyVisualizer;
+
+use Monolog\Formatter\LineFormatter;
+use Monolog\Handler\NullHandler;
+use Monolog\Handler\StreamHandler;
+use Monolog\Logger as MonologLogger;
+
+final class Logger
+{
+    private static ?MonologLogger $logger = null;
+
+    public static function initialize(bool $enable): void
+    {
+        self::$logger = new MonologLogger('dependency visualizer');
+        if ($enable) {
+            $handler = new StreamHandler(getcwd() . '/app.log');
+            $handler->setFormatter(new LineFormatter(null, null, true));
+            self::$logger->pushHandler($handler);
+        } else {
+            self::$logger->pushHandler(new NullHandler());
+        }
+    }
+
+    /**
+     * @param mixed[] $context
+     */
+    public static function info(string $message, array $context = []): void
+    {
+        if (self::$logger === null) {
+            return;
+        }
+        self::$logger->info($message, $context);
+    }
+
+    /**
+     * @param mixed[] $context
+     */
+    public static function error(string $message, array $context = []): void
+    {
+        if (self::$logger === null) {
+            return;
+        }
+        self::$logger->error($message, $context);
+    }
+}
diff --git a/src/Visitor/ClassVisitor.php b/src/Visitor/ClassVisitor.php
index 2fadbc5..bb05637 100644
--- a/src/Visitor/ClassVisitor.php
+++ b/src/Visitor/ClassVisitor.php
@@ -6,6 +6,7 @@
 use Hirokinoue\DependencyVisualizer\ClassManipulator\ClassLoader;
 use Hirokinoue\DependencyVisualizer\Config\Config;
 use Hirokinoue\DependencyVisualizer\DiagramUnit;
+use Hirokinoue\DependencyVisualizer\Logger;
 use PhpParser\Node;
 use PhpParser\Node\Name\FullyQualified;
 use PhpParser\NodeTraverser;
@@ -19,6 +20,7 @@ final class ClassVisitor extends NodeVisitorAbstract
     public function __construct(DiagramUnit $diagramUnit) {
         $diagramUnit->registerVisitedClass();
         $this->diagramUnit = $diagramUnit;
+        Logger::info('instantiate ClassVisitor', ['name' => $diagramUnit->fullyQualifiedClassName()]);
     }
 
     public function enterNode(Node $node) {
@@ -36,6 +38,7 @@ public function enterNode(Node $node) {
 
         $classFile = ClassLoader::create($node);
         if ($classFile->isClass()) {
+            Logger::info('load class', ['name' => $classFile->className()]);
             $ancestors = $this->diagramUnit->ancestors();
             $ancestors[] = $node->toCodeString();