To solve the problem that the pressure of the suction part of a hydraulic pump is dropped unusually, cavitation is generated and the generation of a noise and the destruction of the hydraulic pump are brought, when the rotational speed and rotational acceleration of a motor for driving the hydraulic pump becomes higher.

Pressure drop is prevented by installing a check valve 11 for allowing the flow from a hydraulic tank 10 to a port 1b near the port 1b of a hydraulic pump 1 in the hydraulic closed circuit in which the ports 1a, 1b of the hydraulic pump 1 is communicated with the ports 3a, 3b of a hydraulic actuator 3 with a different pressure receive area by pipe passages 4, 5 and the discharge of a surplus flow rate or the supply of a short flow rate generated in the pipe passage 4, 5 is carried out by control valves 6, 7 by which the pilot pressure of the pipe passage 4, 5 is guided and supplying a fluid to a suction part before a cavitation generation.