To increase starting power of the actuator simply by fitting a fluid system to the forward stage of an input port of the actuator.

In the initial movement of a piston 2 from the initial position, pressure in a primary pressure room 31 is intensified and generated in a secondary pressure room 32 by the difference of areas of a large diameter end 21a of the piston 2 and a small diameter end 22a of the piston 2 and the fluid of the secondary pressure room 32 is outputted to an output port 92, while, during the piston 2 is proceeding to the final end position after passing through a switchover position, a bypass path 61 makes the primary pressure room 31 communicate with the secondary pressure room 32 and acts to output the fluid in the primary pressure room 31 as it is. An actuator with improved starting power can be obtained with such simplified composition because pressure higher than the system pressure can be automatically supplied to the actuator, when the actuator is started, by guiding the system pressure to the primary pressure room 31 and the secondary pressure room 32 is communicated with the actuator.