The etiology of the papillary muscle rupture includes myocardial infarction, trauma, hypertension, myxomatous degeneration, endocarditis etc. We report a case of partial papillary muscle rupture whose etiology was unknown, in a 77-year-old woman. The preoperative catheterization and coronary angiography showed severe mitral regurgitation and no significant coronary stenosis. And we recognized the mass waving into the left atrium in systole with the echocardiogram. At surgery, we repaired the mitral valve by resecting quadrangular areas of the posterior leaflet including the attachment of the torn papillary muscle. Additionally a number 28 Carpentier-Edwards mitral annuloplasty ring was sewn in place. In pathologic specimen, there were focal fibrosis, necrotic muscle, lymphocytes, and no vegetation.