PURPOSE: To shorten the moving distance of a striker and to reduce the size of a device by juxtaposing a male screw part for detecting a position, having a pitch lower than that of a male screw part for a feed, with the outer periphery of a shaft.

CONSTITUTION: When a shaft 15 is rotationally driven with the aid of a motor, a female screw member for a feed is moved by a male screw part for a feed on the shaft 15 and a striker 47 is synchronously moved by a male screw part 46 for detecting a position. In this case, since the pitch of the male screw part 46 for detecting a position is lower than that of the male screw for a feed on a coaxial axis, a moving amount of the striker 47 is reduced to a value lower than that of the female screw member for a feed. Therefore, a pair of position detecting switches 58 and 59 are disposed at the two end positions of a distance shorter than the moving distance of the female screw member for a feed. This constitution reduces the installation spaces of the position detecting switches 58 and 59, simplifies wiring structure therefor, and promotes reduction of the size of the whole of an actuator.