In this paper, we report four cases in which active assist circulation was performed using IABP and ECC with aortic cannulation at the ascending aorta to improve left ventricular function that had fallen into severe LOS at or after weaning from cardio pulmonary bypass (CPB) during cardiac surgery. Case 1: 51-year-old female, thrombectomy (LA)+MVR+single ACBG. Case 2:68-year-old female, AVR. Case 3: 51-year-old female, MVR + repair for postoperative rupture of posterior left ventricular wall. Case 4: 48-year-old male, emergency double ACBG. We performed active assist circulation on them at (Cases 1, 2 and 4) or after (Case 3) weaning from CPB. The condition of patients who require active assist circulation is very serious. Thus, it is difficult to treat them completely. When active assist circulation is continued for a long time, hemolysis, infection, ischemia of lower limb, multiple organ failure, etc., may develop, and the patient's prognosis will worsen. Consequently, for severe LOS patients at or after weaning from CPB, efforts must be made to solve these problems and active treatment which can be done at any institute must be given prior to the implementation of an artificial heart.