PURPOSE: To prevent the pumps and hydraulic equipment from being damaged by setting a direction selecting valve at a cut-off position when the oil temperature is low and allowing the oil to circulate in a hydraulic circuit.

CONSTITUTION: A tank oil temperature sensor 34 is installed into a tank 33, and the output supplied from the tank oil temperature sensor 34 is input into a controller 43. When the oil temperature is low, a direction selecting valve 4 is set at a cut-off position, and the first and the second pilot valves 30 and 31 are opened to allow oil to circulate in a hydraulic circuit. Since the temperature of the oil can be raised, hydraulic equipment is operated when the temperature of the oil in the hydraulic circuit is low, and pumps and hydraulic equipment can be prevented from being damaged by the generation of cavitation.