We report a case of iatrogenic pneumothorax in which chest tube placement was avoided by continuous intraoperative evaluation with transthoracic ultrasonography. A 53-year-old man had undergone a subsegmentectomy. While attempting to place a central venous catheter in the right internal jugular vein after the induction of anesthesia, we identified gas absorption during the puncture and suspected a pneumothorax. Chest X-ray revealed an ~5-mm collapse of the right lung apex. Tension pneumothorax was a concern during surgery because of the long-term positive pressure ventilation, but we decided to start the operation without preventative chest tube placement. During the operation, we regularly observed the midclavicular line of the second intercostal space using ultrasound. The operation was completed uneventfully. In this case, we effectively utilized ultrasound and avoided preventive chest tube placement and the associated complications. Transthoracic ultrasonography could be performed easily and continuously during surgery and was effective for evaluating the progression of an intraoperative pneumothorax.