An 83-year-old woman with unstable angina pectoris underwent percutaneous coronary intervention (PCI) of the left circumflex artery, and her condition improved. However, on the eighth day after PCI, she went into a stated shock, and echocardiogram confirmed a large amount of pericardial effusion. Pericardiocentesis was immediately performed, and bloody pericardial effusion was drained. Cardiac rupture was suspected, although the cause was unknown. Emergency sternotomy was performed, and blow out type cardiac rupture in the center of a thumb-sized infarction was found at the area of the obtuse marginal branch. The ruptured left ventricular wall was successfully closed with 2 mattress sutures because the infarcted area was relatively small. Postoperative course was good, and she was discharged on the 25th postoperative day. In this case, the cause of cardiac rupture was thought to be a small branch of the left circumflex artery, which was occluded during PCI. This is one of the rare but important mechanisms of cardiac tamponade after PCI.