This report presents the first case of anterolateral papillary muscle rupture caused by a diagonal branch occlusion only. Although the patient was in shock on admission, he was successfully treated by emergency surgery after hemodynamic stabilization by a percutaneous cardiopulmonary support system. This case implies that the anterolateral papillary muscle has a single blood supply and that it can rupture by an occlusion of only one diagonal branch even after balloon angioplasty.