A 76-year-old female was admitted to our hospital due to anterior chest pain and dyspnea. Mitral regurgitation due to prolapse of the posterior leaflet was detected by UCG. After admission, massive gastric hemorrhage was observed. Because hemostatic therapy using endoscopy was not effective, partial gastrectomy was performed. The origin of the hemorrhage, an acute gastric ulcer, was located on the side of the minor curvature of the corpus ventriculi. After gastrectomy, the patient underwent medical treatment using an IABP, but the left heart failure was not reduced, and the pulmonary edema worsened. At 18 hours after gastrectomy, MVR was performed. The cause of regurgitation is torn chordae of the posterior leaflet. The postoperative course was good, and the patient is doing well in NYHA class 1. This case is the first report of acute mitral insufficiency associated with acute gastric lesion in Japan.