Quadricuspid aortic valve (QAV) is a rare congenital heart disease. A 66-year-old man was admitted for investigation of a cardiac murmur. Transthoracic echocardiography (TTE) demonstrated severe aortic regurgitation (AR). QAV was diagnosed on preoperative transesophageal echocardiography (TEE). During the operation, the accessory cusp was found to be located between the left and right coronary cusps. The patient had an aortic valve replacement using a mechanical prosthetic valve, due to difficulty in repairing the valve. The patient's postoperative course was uneventful.