Hypoxemia remains one of the most common postoperative problems. It is difficult to detect but potentially lethal, especially in combination with other complications. A case is presented of persistent hypoxemia resulting from pneumothorax in a 44-year-old male patient having surgery for the pancreatic head cancer under general anesthesia. He was catheterized in the right subclavian vein by the infraclavicular approach for the measurement of CVP on the first preoperative day and developed pneumothorax subsequently. This complication was thought to have been aggravated due to inattentive positive pressure ventilation and diffusion on N2O during anesthesia. This patient recovered from postoperative hypoxemia after needle aspiration of air from the pleural cavity without any other problem in the recovery room.