PURPOSE: To surely displace only an inner side piston of small diameter, even when external diametric dimension of an actuator is decreased in order to attain forming into small size, lightening weight and improving an operating speed.

CONSTITUTION: In case of displacing an inner side piston 37a of small diameter having a small pressure receiving area relating to an outer side piston 31a, compressed air is supplied only in a short time not only to the second port 40a but also to the first port 35a. As a result, a rod 10 is pressed up with sufficiently large force by the outer side piston 31a having a large pressure receiving area, to separate a valve body 9 from an opening peripheral edge of a circulating hole 8. After the valve body 9 is separated from the opening peripheral edge, supplying compressed air to the first port 35a is stopped, to continue only supplying compressed air to the second port 40a.