PURPOSE: To suppress leakage to a minimum level required by providing a valve body that responds to the difference in pressure between a pair of fluid chambers on an orifice route provided between the respective fluid chambers, and by providing a regulating valve that is opened or closed according to the difference in pressure.

CONSTITUTION: A hydraulic cylinder 11 has a pair of hydraulic chambers 15, 16 (fluid chambers) that are partitioned by a piston 13, and are connected with a control valve 12 by passages 14a, 14b. Orifice passages 17, 18 provided with fixed orifices 17a, 18a are provided between the pair of hydraulic chambers 15 and 16, and a regulating valve 19 is provided on the passages 17, 18. Since the regulating valve 19 is closed until the pressure difference between the pair of fluid chambers 15 and 16 reaches a preset value, while it is opened when the pressure difference reaches the preset value, fluid leakage through the orifice passages 17, 18 can be suppressed to a minimum level required, and the rigidity of an actuator can thus be ensured.