A 57-year-old man underwent partial resection of the right diaphragm with invasive thymoma dissemination. Fifteen days after surgery, he suddenly developed right-sided chest pain with dyspnea and was raced to the hospital. Chest computed tomography (CT) showed a massive right hemothorax, and emergency surgery was performed due to hemodynamic shock. Thoracotomy revealed intercostal artery bleeding from a pinhole injury caused by a non-absorbable monofilament suture tip of the diaphragm. Hemothorax after thoracic surgery is a serious complication, and surgical materials with sharp edges may cause bleeding by contact with intrathoracic organs. Therefore, these materials should be used with caution and covered or cut as needed.