diff --git a/embulk-junit5-engine/src/main/java/org/embulk/junit5/engine/EmbulkPluginTestEngine.java b/embulk-junit5-engine/src/main/java/org/embulk/junit5/engine/EmbulkPluginTestEngine.java
index 50b0517..eba4e2c 100644
--- a/embulk-junit5-engine/src/main/java/org/embulk/junit5/engine/EmbulkPluginTestEngine.java
+++ b/embulk-junit5-engine/src/main/java/org/embulk/junit5/engine/EmbulkPluginTestEngine.java
@@ -19,6 +19,8 @@
 import java.lang.reflect.Method;
 import java.util.Optional;
 import org.embulk.junit5.api.EmbulkPluginTest;
+import org.junit.platform.commons.logging.Logger;
+import org.junit.platform.commons.logging.LoggerFactory;
 import org.junit.platform.engine.EngineDiscoveryRequest;
 import org.junit.platform.engine.ExecutionRequest;
 import org.junit.platform.engine.TestDescriptor;
@@ -29,8 +31,17 @@
 import org.junit.platform.engine.support.hierarchical.OpenTest4JAwareThrowableCollector;
 import org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService;
 import org.junit.platform.engine.support.hierarchical.ThrowableCollector;
+import org.junit.platform.launcher.LauncherDiscoveryRequest;
 
 public final class EmbulkPluginTestEngine extends HierarchicalTestEngine<EmbulkPluginTestEngineExecutionContext> {
+    public EmbulkPluginTestEngine() {
+        super();
+        final Class<?> klass = this.getClass();
+        this.klassLoader = klass.getClassLoader();
+        logger.info(() -> "Initializing EmbulkPluginTestEngine@" + Integer.toHexString(this.hashCode()));
+        logger.info(() -> "EmbulkPluginTestEngine's ClassLoader: " + this.klassLoader.toString());
+    }
+
     /**
      * Returns {@code "embulk-junit5-engine"} as the engine ID.
      */
@@ -48,15 +59,14 @@ public final String getId() {
      */
     @Override
     public TestDescriptor discover(final EngineDiscoveryRequest discoveryRequest, final UniqueId uniqueId) {
-        System.out.println("EngineDiscoveryRequest: " + discoveryRequest.toString());
-        if (discoveryRequest instanceof org.junit.platform.launcher.LauncherDiscoveryRequest) {
-            final org.junit.platform.launcher.LauncherDiscoveryRequest launcherDiscoveryRequest =
-                    (org.junit.platform.launcher.LauncherDiscoveryRequest) discoveryRequest;
-            System.out.println("  EngineFilters: " + launcherDiscoveryRequest.getEngineFilters());
-            System.out.println("  PostDiscoveryFilters: " + launcherDiscoveryRequest.getPostDiscoveryFilters());
+        logger.trace(() -> "EngineDiscoveryRequest: " + discoveryRequest.toString());
+        if (discoveryRequest instanceof LauncherDiscoveryRequest) {
+            final LauncherDiscoveryRequest launcherDiscoveryRequest = (LauncherDiscoveryRequest) discoveryRequest;
+            logger.trace(() -> "  EngineFilters: " + launcherDiscoveryRequest.getEngineFilters());
+            logger.trace(() -> "  PostDiscoveryFilters: " + launcherDiscoveryRequest.getPostDiscoveryFilters());
         }
-        System.out.println("  ConfigurationParameters: " + discoveryRequest.getConfigurationParameters());
-        System.out.println("UniqueId: " + uniqueId.toString());
+        logger.trace(() -> "  ConfigurationParameters: " + discoveryRequest.getConfigurationParameters());
+        logger.trace(() -> "UniqueId: " + uniqueId.toString());
 
         final EmbulkPluginTestEngineDescriptor engineDescriptor = new EmbulkPluginTestEngineDescriptor(uniqueId);
 
@@ -72,12 +82,7 @@ public TestDescriptor discover(final EngineDiscoveryRequest discoveryRequest, fi
 
             // final Class<?> testClass = classSelector.getJavaClass();  // Not to get the Java class directly!
             final String testClassName = classSelector.getClassName();
-            final Class<?> testClass;
-            try {
-                testClass = Class.forName(testClassName);  // <= TODO: Load it under PluginClassLoader !
-            } catch (final ClassNotFoundException ex) {
-                throw new RuntimeException(ex);
-            }
+            final Class<?> testClass = findOrLoadClassFrom(this.klassLoader, testClassName);;
 
             final TestDescriptor classDescriptor =
                     new ClassTestDescriptor(uniqueId.append("class", testClass.getName()), testClass);
@@ -129,4 +134,22 @@ public final Optional<String> getArtifactId() {
     protected EmbulkPluginTestEngineExecutionContext createExecutionContext​(final ExecutionRequest request) {
         return new EmbulkPluginTestEngineExecutionContext();
     }
+
+    private static Class<?> findOrLoadClassFrom(final ClassLoader klassLoader, final String name) {
+        final Class<?> foundClass = LoadedClassFinder.findFrom(klassLoader, name);
+        if (foundClass != null) {
+            logger.info(() -> "\"" + name + "\" has been already loaded: " + foundClass.toString());
+        }
+
+        logger.info(() -> "\"" + name + "\" has not been loaded.");
+        try {
+            return Class.forName(name);  // <= TODO: Load it under PluginClassLoader !
+        } catch (final ClassNotFoundException ex) {
+            throw new RuntimeException(ex);
+        }
+    }
+
+    private static final Logger logger = LoggerFactory.getLogger(EmbulkPluginTestEngine.class);
+
+    private final ClassLoader klassLoader;
 }
diff --git a/embulk-junit5-engine/src/main/java/org/embulk/junit5/engine/LoadedClassFinder.java b/embulk-junit5-engine/src/main/java/org/embulk/junit5/engine/LoadedClassFinder.java
new file mode 100644
index 0000000..e14f7de
--- /dev/null
+++ b/embulk-junit5-engine/src/main/java/org/embulk/junit5/engine/LoadedClassFinder.java
@@ -0,0 +1,29 @@
+/*
+ * Copyright 2024 The Embulk project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.embulk.junit5.engine;
+
+class LoadedClassFinder extends ClassLoader {
+    private LoadedClassFinder(final ClassLoader inner) {
+        this.inner = inner;
+    }
+
+    static Class<?> findFrom(final ClassLoader classLoader, final String name) {
+        return (new LoadedClassFinder(classLoader)).findLoadedClass(name);
+    }
+
+    private final ClassLoader inner;
+}