To provide an actuator which can be made compact in configuration and can secure high air tightness in a cylinder prior to the actuation and the smooth forwarding movement of a piston rod at the time of actuation.

The actuator A includes a configuration which makes the piston rod 47 in the cylinder 22 move forward by allowing gas for operation to flow therein. A support rod 55 of the piston rod 47 is provided with a shaft 56 extending from a piston and a head 57. The cylinder 22 includes a body 23 and a tip wall 31 clogging the tip side of the body 23, and includes an insertion hole 32 capable of penetrating the shaft 56 at the tip wall 31. The support rod 55 causes an O-ring 59 interposed between the tip wall 31 and itself, and then seals the inside of the cylinder 22 to be suitable for the circumference of the shaft 56 in a state before operation. The O-ring 59 is made to move accompanied by the forward movement of the piston rod 47 at operation.