A 73-year-old woman was referred to our hospital to investigate dilatation of an aortic arch which had been detected by a chest roentgenogram and severe aortic valve regurgitation detected by echocardiography. On admission, a computed tomography scan of the chest showed a large fusiform ascending aortic aneurysm. She had not shown any symptoms such as headache or polymyalgia rheumatica and had no significant coronary atherosclerosis. She underwent aneurysmectomy and reconstruction of the ascending aorta using cardiopulmonary bypass without aortic valve replacement, and pathological examination of the aneurismal wall revealed giant cell arteritis (GCA). Preoperatively, she did not have any temporal pain, and no signs of inflammation were detected serologically. Postoperatively, aortic valve regurgitation improved and she did well. However, three months after the surgery, she died suddenly due to the rupture or dissection of aorta. In the Japanese population, GCA is reportedly a rare cause of aortic aneurysm. However, retrospective studies show that GCA affects the aorta and that thoracic aortic aneurysm is a possible complication of GCA. In cases of the thoracic aortic aneurysms with unknown etiology, there is a possibility that GCA is the cause of the aortic aneurysm.