Preservation of subvalvular mitral apparatus and maintenance of continuity between structures and annulus is recognized, and widely accepted as a significant factor for avoiding impairment to ventricular function and preventing left ventricular rupture during mitral valve replacement. However, we encountered a patient who developed posteromedial papillary muscle rupture following chordal sparing mitral valve replacement. The patient was a 67-year-old man who underwent mitral valve replacement with a porcine bioprosthesis 29M for acute mitral valve insufficiency due to several spontaneous chordal ruptures. The subvalvular apparatus of both leaflets was retained, the center of the anterior leaflet was excised elliptically, and the entire posterior leaflet was preserved. Although his postoperative course was uneventful, the transthoracic echocardiogram showed a floating structure prolapsing through the aortic valve in the left ventricle synchronizing with the cardiac cycle. The severed papillary muscle was removed successfully via an aortotomy through the native aortic valve on the 57th day after the first surgery. The patient recovered with no events. Surgeons should consider avoiding an excessive tension on the preserved chordae and delivering a cardioplegia sufficiently and uniformly during mitral valve replacement.