We experienced a case of mitral incompetence due to rupture of anterolateral papillary muscle in a 56-year-old male who complained of abdominal pain and mild dyspnea after being struck by car. Clinical manifestation immediately following injury was minimal, but heart failure progressed rapidly 3 days after injury. Echocardiopraphic evaluation revealed ruptured anterolateral papillary muscle and severe mitral regurgitation on color flow imaging. There was no evidence of coronary artery disease on coronary angiography. Operation revealed that the haed of anterolateral papillary muscle was torn out of its insertion. Mitral valve replacement with mechanical prosthesis was performed on the 50th day after injury.