diff --git a/src/main/java/org/codelibs/fess/thumbnail/playwright/PlaywrightThumbnailGenerator.java b/src/main/java/org/codelibs/fess/thumbnail/playwright/PlaywrightThumbnailGenerator.java
index 632448c..3b1791a 100644
--- a/src/main/java/org/codelibs/fess/thumbnail/playwright/PlaywrightThumbnailGenerator.java
+++ b/src/main/java/org/codelibs/fess/thumbnail/playwright/PlaywrightThumbnailGenerator.java
@@ -184,7 +184,7 @@ public boolean generate(final String thumbnailId, final File outputFile) {
         });
     }
 
-    protected void createScreenshot(final String url, final int width, final int height, final File outputFile) {
+    protected synchronized void createScreenshot(final String url, final int width, final int height, final File outputFile) {
         final Page page = worker.getValue4();
         File tempPngFile = null;
         try {