Quadricuspid aortic valve is a rare congenital cardiac anomaly that causes aortic regurgitation. We used transthoracic echocardiography to diagnose a case of quadricuspid aortic valve. The patient was a 76-year-old man with dyspnea and epigastralgia and who had been examined by a general practitioner. He was subsequently hospitalized at this institution when heart failure was indicated. Transthoracic echocardiography showed left ventricular dilatation and systolic dysfunction. The aortic valve was quadricuspid and had an accessory cusp between its right and the left cusps, thus causing severe aortic regurgitation resulting from coaptation failure. The valve was replaced when aortography also revealed severe aortic regurgitation. This case is valuable for having provided the basis for a definitive diagnosis of quadricuspid aortic valve from findings obtained using transthoracic echocardiography.