To cool the hydraulic oil by an oil cooler corresponding to the rise of a temperature of the hydraulic oil even in non-operation of a hydraulic actuator.

The oil cooler 8 is mounted at a downstream side of first and second center bypass oil passages E, F where the discharged pressure oil of first and second pressure oil pumps 1, 2 flows to an oil tank 4 in an unloaded state, and a flow rate of the first and second oil pressure pumps is increased corresponding to the rise of the temperature of the hydraulic oil in non-operation of the hydraulic actuator on the basis of a detecting signal from the hydraulic oil temperature detecting sensor 11, whereby the oil supply amount to the oil cooler is increased to achieve the speed-up in cooling the hydraulic oil.