A case of mitral valve aneurysm associated with infective endocarditis is reported. Two-dimensional echocardiography revealed a saccular structure in the anterior leaflet that bulged into the left atrium throughout the cardiac cycle. During operation, the vegetation on the commissure of the right and left aortic leaflet and a 3-mm perforation on the noncoronary leaflet were found. The mitral valve and aortic valve were replaced with mechanical prosthesis. Pathology of the excised valves showed inflammation. For this patient, we considered that the infected aortic regurgitant jet striking the ventricular surface of the anterior mitral leaflet could be the mechanism of the leaflet aneurysm.