The patient was a 74-year-old man who was brought to the emergency room with severe chest pain and shock. Transthoracic echocardiography showed moderate pericardial effusion, and contrast-enhanced computed tomography (CT) showed a dilated ascending aorta with hematoma. However, no evidence of an intimal flap in the aorta was found. Bloody pericardial effusion was suggested by the CT attenuation value ; therefore, type A aortic dissection was highly suspected. At surgery, an extramural hematoma was observed on the ascending aorta. An 8-mm dehiscence that had penetrated the adventitia was identified just above the commissure between the right and left coronary cusps of the aortic valve, without dissection in the ascending aorta, and thus spontaneous aortic root rupture was diagnosed. The dehiscence was closed directly with a mattress suture from outside of the sinus, and the dilated ascending aorta was replaced. The patient's postoperative course was unremarkable, and he was discharged 14 days after surgery.