PURPOSE: To save power consumption by conducting flow control of a pump using a variable speed motor with the minimum required power over a wide area.

CONSTITUTION: A calculating device 23 is to convert the discharge pressure of an auxiliary pump 20 and the numbers of revolutions of a main pump 17 and the pump 20 to output signals, by means of the input signals from a flow rate instituting device 24 and a discharge pressure instituting device 25. These output signals serve to control the opening of a regulator valve 27 provided at a divergent pipe 3 by a regulation meter 26 so that the discharge pressure of the auxiliary pump 20 is held at a preset value. The numbers of revolutions of main 17 and auxiliary 20 pumps are sensed by speed sensors 28, 29 in direct coupling with motors 19, 22 and speed-controlled by variable speed driving devices 18, 21, respectively so that the set value is attained by the calculating device 23. Thus the rate of flow of a pump is controlled using a variable speed motor, and it gives possibility of saving power consumption.