We report a case of severe mitral regurgitation due to partial rupture of an anterior papillary muscle. A 63-year-old man was admitted to a hospital with heart failure. He was treated with diuretic agents effectively. Echocardiography demonstrated severe mitral regurgitation with prolapse of posterior leaflet and small mass-like structure on the prolapsed segment that was diagnosed the thickened leaflet. Coronary angiography revealed total occlusion of left anterior descending artery (LAD) filled with good collateral from right posterior descending artery and severe diffuse stenosis of circumflex artery (Cx). The patient underwent surgery on the 33rd day after admission with heart failure. At surgery, we recognized rupture of one of the heads of anterior papillary muscle that was entangled in chordae of the prolapsed segment. Mitral valve repair and coronary revascularization to LAD and Cx was successfully performed. His postoperative course was uneventful, and he was discharged on the 28th postoperative day.