PURPOSE: To compact the device by providing an assist means for assisting the stroke of a piston with fluid pressure from a pressure source, when a valve means for isolating an operation system from the pressure source, in addition to the stroke determined with a working pressure determination means.

CONSTITUTION: When the subject actuator is applied to a brake control actuator, a variable displacement piston 9 is so coupled to a pressure control cylinder 8 as to be freely slidable, and both ends thereof are closed with end covers 10 and 11, thereby cellularizing sealed chambers 14 and 15. The sealed chamber 14 is connected to a master cylinder pressure pipe line 4 via shut-off valves 20 and 21 sequentially, while the sealed chamber 15 is connected to the aforesaid pipe line 4 via a shut-off valve 21. Also, an actuator body 1 is fitted with a fluid pressure assist cylinder 50 at the end of a piston rod 6, and a piston 51 fitted to the end of the piston rod 16 is coupled to the cylinder 50, thereby cellularizing an assist fluid chamber 52. in addition, constitution is so made that master cylinder liquid pressure can be introduced to the aforesaid chamber 52 via tandem type integrated solenoid valves 55 and 56, when pressure reduction control is demanded.