An aortic aneurysm was incidentally diagnosed in a 75-year-old woman during a thorough examination for other diseases. She had a history of total arch replacement( TAR) for aortic arch aneurysm 17 years previously. Contrast-enhanced computed tomography( CT) revealed a proximal aortic aneurysm with a maximum diameter of 67 mm protruding to the lateral side. She was treated by elective ascending aortic replacement. The resected aneurysm was not a pseudoaneurysm, but a true aneurysm. The etiology of this aneurysm might be long-term hemodynamic stress from the left ventricle and inadequate blood pressure control in addition to the anatomical position of the proximal residual aorta after first surgery. Therefore, to prevent aneurysm formation, it is important to replace the ascending aorta as proximally as possible at first surgery and to continue strict postoperative blood pressure control.