To decrease the number of part items so as to attain miniaturization of a device, by reducing a pump ripple at exhaust time while efficient pump operation can be made with a small change of torque when a pump is driven.

In an output shaft 5 of a motor 2, a crank pin 6 is eccentrically mounted. In a peripheral direction with the output shaft 5 serving as the center, an air bag 17 is equally arranged. In an operating pin 15 in an end part of an operating arm 13 turnably supported to the crank pin 6, a piston 18 integrally provided in the air bag 17 is bearing supported. By rotation with the output shaft 5 of the crank pin 6 serving as the center, the air bag 17 performs air suction/discharge successively by a fixed phase difference.