PURPOSE: To hold constant the discharge of a rotary pump irrespective of hydraulic loads by detecting the rotating speed of an electric motor for driving the rotary pump, and controlling the power supply to the electric motor according to the result of comparison of the detected rotating speed with a desired rotating speed.

CONSTITUTION: A rotating speed detector 3 is provided on the rotary shaft of an electric motor M directly connected to a vane pump and its detection signal is input to a comparing portion 4 to compare the detected rotating speed with a desired rotating speed. The deviation obtained between the rotating speeds is input to a control portion 5 to control the power supply to the electric motor M; i.e., when the detected rotating speed is higher than the desired rotating speed, the power supply is reduced, whereas when the detected speed is lower than the desired speed the power supply is increased. The rotating speed of the electric motor M is thereby held constant and the output of the vane pump is also held constant irrespective of hydraulic loads.