Pneumothorax is a rare and potentially serious complication that can be occurred during laparoscopic surgery. This is a case report of a 34-year-old man who underwent elective laparoscopic partial hepatectomy under general anesthesia. About 45 min after carbon dioxide pneumoperitoneum, hypotension, tachycardia, decreased oxygen saturation, increased peak inspiratory pressure, increased end-tidal CO₂ tension, hypercapnia, and hypoxemia were suddenly developed. Altering for the worse vital sign was owing to tension pneumothorax by accidental CO₂ influx through inadvertently perforated diaphragm. Repairing diaphragm under open laparotomy, positive manual bagging was effectively applied enough not to require surgical drainage. Therefore, closed observation and careful monitoring is essential to prevent tension pneumothorax during laparoscopic surgery.