PURPOSE: To control a suction stage of blood into a blood receiving space so as to maximize the circulation rate of the blood per unit time and to decrease the burden on a living body by calculating the average flow rate of the blood per unit time by a calculating means and setting an integral set value in such a manner that the calculated value attains the highest value.

CONSTITUTION: A pump 1 is equipped with a flow velocity detecting means 13 for detecting the flow velocities of the blood entering into the blood receiving space 5 from a blood receiving port 7, low-pressure fluid sources 32n, 35 and high-pressure fluid sources 32p, 35. An operating fluid space 6 is selectively connected to the low-pressure fluid sources 32n, 35 and the high-pressure fluid sources 32p, 35 by connection changeover means (31n, 31p). Flow velocities are integrated to calculate the integral values by integrating means while the working fluid space 6 is held connected to the low-pressure fluid sources 32n, 35. The average flow rate of the blood per unit time is then calculated. The integral set value is updated in such a manner that the calculated value attains the highest value. The working fluid space 6 is connected to the high-pressure fluid sources 32p, 35 by a first control means when the integral value attains the integral set value.