PURPOSE: To increase a control range by a method wherein, in a device which controls a delivery amount of a variable delivery amount pump based on a control signal from a controller, an auxiliary pump is connected through a control valve to a control cylinder for regulating a delivery amount.

CONSTITUTION: A swash plate type variable delivery amount pump 3 driven with the aid of a motor 11 has a delivery amount regulated by controlling the inclination angel of the swash plate by means of a control cylinder 4. The delivery port of the pump 3 is connected to a main circuit 14 branched according to the number of actuators, and the main circuit 14 is connected to an actuator A1 through a switching valve 16. In this case, an auxiliary pump 6 driven with the aid of a motor 20 is provided, and the delivery port thereof is connected to the control cylinder 4 through a control valve 5. The control valve 5 is controlled by a controller 2 based on detecting values from a swash plate angle sensor 12 and a pressure sensor 17 and a set value by a setter group 18.