PURPOSE: To prevent an actuator from running away and a cavitation from being generated even if the counter load is generated by enabling a system to cope with the situation even through the direction of load is reversed.

CONSTITUTION: Meter-in control is performed while judging the differential pressure across first and second three-way valves V1, V2 by signals from a self pressure sensor 29 and load pressure sensors 30, 31. Moreover, the direction of load is judged on a meter-out side on the basis of pressure deviation on the meter-in side therefrom and the openings of the first and second three-way valves V1, V2 are controlled according to the direction of load.