A 76-year-old woman with a diagnosis of hypertrophic obstructive cardiomyopathy was referred to our hospital's surgical department. Her echocardiogram revealed diffuse left ventricular hypertrophy, moderate mitral valve regurgitation with systolic anterior motion of the mitral valve, and left ventricular obstruction with a peak outflow gradient of 108 mm Hg. We performed a transaortic rectangular septal myectomy with an incision at a width, depth, and length of 1 cm, 1 cm, and 3 cm, respectively. However, the transesophageal echocardiogram revealed residual left ventricular obstruction and systolic anterior motion, and we subsequently replaced the mitral valve with a mechanical valve. The patient's postoperative course was uneventful, and the peak outflow gradient decreased to 15 mm Hg. Although transaortic septal myectomy is the most common surgery currently used for hypertrophic obstructive cardiomyopathy, mitral valve replacement should remain an option in patients with diffuse left ventricular hypertrophy who fail to improve after myectomy alone.