A 62-year-old female patient was scheduled for emergent explo-laparotomy under general anesthesia because of traffic accident. Preoperative checked chest x-ray showed multiple fractures of the ribs but didn't show the sign of pneumothorax or hemothorax. she had no dyspnea. Compared with previous value(PaO2; 210.5 mmHg), Arterial PO2 value(PaO2; 143.0 mmHg) was reduced significantly two hours after starting general anesthesia. Chest x-ray was taken in the operating room, which showed pneumothorax in the right side lung. The patient was treated with immediate closed thoracostomy. She recovered uneventfully three days later with complete resorption of the pneumothorax.