To facilitate simpler construction of a device to be smaller and lighter for actuating an operating rod with slight force and improving a degree of freedom in pull-around and installation and workability.

The actuator comprises a first piston 2 to which the operating rod 3 is connected, a second piston 8 for reducing or increasing a sealed space to compress or expand air, a cylinder 4 slidably storing the first piston 2, a pressure container 6 slidably storing the second piston 8, and a fluid channel 8 communicating the cylinder 4 with the pressure container 6, whereby the sealed space is formed for sealing the air 5 as operating fluid. The sealed space is compressed or expanded by the sliding operation of the second piston 8 and a change in the pressure of the air 5 with the compression and expansion is transmitted to the first piston 2, permitting reciprocating motion of the operating rod 3 connected to a side opposite to the sealed space of the first piton 2 to the inside/outside of the cylinder 4.