A 67-year-old woman with chronic cholecystitis was scheduled to have laparoscopic cholecystectomy under general anesthesia. About 5~10 min after the CO2 intraperitoneal insufflation, the peak airway pressure gradually increased from 15 cmH2O to 27 cmH2O, the end-tidal CO2(EtCO2) from 32 mmHg to 56 mmHg. The SpO2 decreased from 100% to 96%, and blood pressure from 135/80 mmHg to 80/52 mmHg. A right side tension pneumothorax was confirmed and a drainage tube was placed in the right pleural cavity. As the continuous gas leakage from the drainage tube was noted, even as ventilation was withheld, the diaphragm was carefully examined and a porous diaphragm was found. These defects were then patched with biomedical materials. The operation was finished uneventfully. It was concluded that in a patient with a tension pneumothorax during laparoscopic surgery, a diaphragm defect should be taken into consideration.