PURPOSE: To apply a wide range of loads accurately to a hydraulic actuator.

CONSTITUTION: There is provided to a rod 2 inserted to a cylinder 1 a large piston 3a and a small piston 3c which have different pressure bearing areas, and line valves 6a, 6c for line changeover use are fitted to pressure lines 5a, 5c respectively to connect the respective pistons 3a, 3c to a pressure pump 4, and at the same time, both side chambers of the large piston 3a and the small piston 3c are respectively communicated through balance valves 7a,7c.