We report a rare case of surgical repair of an aneurysm of the membranous septum and mitral valvoplasty. A 57-year-old man suffered from an aneurysm of the membranous septum and mitral regurgitation. We resected the aneurysm completely, closed the defect with a patch in order to prevent further enlargement and consequent complications, and performed mitral valvoplasty using a flexible Duran ring. A postoperative cardiac catheterization indicated that there was no aneurysm of the membranous septum, ventricular septal defect or mitral regurgitation.