Cardiac herniation is a rare, but highly lethal complication of intrapericardial pneumonectomy for progressive carcinoma of lung, and occurs in early postoperative period. Our patient with this complication presented sudden onset of severe hypotension and cyanosis after several ventricular premature beats. We suspected cardiac herniation as we observed dilatation of cervical veins and rightward deviation of heart sound. The patient was immediately positioned on his left side and systemic blood pressure increased soon. After verification of the incident by chest X-ray film, emergency operation was performed. Fortunately, vital signs were stable during and after reoperation and the patient could survive.