An extracorporeal blood circulation system including a line for withdrawing the venous blood from the patient, an artificial lung provided on the line, a reservoir for the blood withdrawn, a blood supply line for sending out the blood from the reservoir to the artery of the patient, and a blood supply pump provided on the blood supply line and serving as an artificial heart. The amount of blood to be supplied is automatically controlled to maintain the arterial pressure of the patient in a suitable required range. The blood supply pump is stopped when the arterial pressure of the patient measured is above a predetermined upper limit value, while the pump is driven when the arterial pressure has lowered to a level below a lower limit value. The amount of blood to be withdrawn is also controlled automatically to keep the central venous pressure of the patient approximately constant. The blood withdrawing line has a blood withdrawing pump. The reservoir is connected to the inlet side of this pump by a shunt line having a valve which is usually opened. The valve is closed when the central venous pressure exceeds a predetermined upper limit level. A vertical overflow tube detects the central venous pressure exceeding the upper limit level.