A 78-year-old woman with mild dementia was found unconscious by her family. She was transported by an ambulance to our emergency room (ER). Initially, she was comatose and in a state of shock. The echocardiographic findings suggested cardiac tamponade by hematoma. Computed tomography also showed tamponade without aortic dissection. After imaging, she went into cardiac arrest, was returned to the ER, and tracheal intubation and left thoracotomy for pericardiotomy were performed. A return of spontaneous circulation was obtained by following this procedure. Bleeding from a rupture of the left cardiac free wall was confirmed, and the rupture was closed with TachoSil®. After closing the thoracotomy, electrocardiography revealed ST elevation in the precordial leads. Subsequently, placement of an indwelling intra-aortic balloon pump and coronary angiography (CAG) were performed. CAG showed an occlusion of the anterior interventricular branch and circumflex branch of the left coronary artery. She underwent conservative therapy in a coronary care unit. Finally, after obtaining hemodynamic stability and baseline mental status, she was transferred to another medical facility. We herein report a rare case involving the successful treatment of cardiac tamponade due to rupture of the heart performing an open-chest pericardiotomy and additionally discuss the key points for obtaining a favorable outcome.