A 69-year-old woman complained of general malaise. Chest X-ray film revealed massive left-sided pleural effusion and CT detected a giant aneurysm of the aortic arch. A diagnosis of ruptured aortic arch aneurysm was made and she underwent total arch replacement as emergency surgery. The intraoperative diagnosis was not a ruptured arch aneurysm but an arch aneurysm penetrating into the left pulmonary artery. The postoperative course was uneventful and the patient was discharged home on the 16th postoperative day.