We report a case of a 68-year-old woman with a large left atrial myxoma occluding the mitral valve. The tumor was diagnosed by preoperative echocardiogram. During the operation and after the myxoma resection, a severe mitral valve regurgitation, which was not observed during the preoperative echocardiogram, was noticed. Mitral valve defect was corrected by an annuloplasty ring, and the patient had an uneventful recovery. Careful evaluation of the mitral valve during a myxoma resection operation is recommended.