A 47-year-old man who had undergone surgery of a dissecting thoracic aortic aneurysm was admitted for a localized, dissecting abdominal aortic aneurysm. Preoperative coronary arteriogram incidentally showed a coronary artery aneurysm (CAA) of the left main trunk. Prior to the operation for abdominal aortic aneurysm, that for the CAA was performed under standard cardiopulmonary bypass conditions with antegrade and retrograde coronary sinus cold blood cardioplegia. A longitudinal incision of the aneurysmal wall was made for the removal of a small thrombus and, the ostia of the left anterior descending artery (LAD), circumflex artery (LCX), intermediate artery (IM) and main trunk were closed from the inside of the aneurysm. The aneurysmal wall was closed without resection of the whole aneurysm. Finally, coronary artery bypass grafting to LAD, LCX and IM was performed. Postoperative course was uneventful with patency of all grafts. The retrograde coronary sinus cardioplagia is effective to prevent thromboembolism of distal coronary arteries due to the antegrade cardioplagia.