PURPOSE: To provide an actuator which is constituted to not only eliminate an expansion part to partially expand when devices are brought into an unit and reduce a space and simplify holding structure but also prevent the generation of noise and vibration and be used in a device needing quietness, and reduce a cost.

CONSTITUTION: An actuator comprises a drive shaft 2 consisting of a hollow shaft and having a female screw part 6 formed in an inner peripheral part; a screw shaft 3 inserted in the drive shaft 2 and joined in a screwed-in state with the female screw part 6; and a detent means to effect detention of rotation of the screw shaft 3. The drive shaft 2 and the screw shaft 3 to effect linear advance movement are positioned on the same axis (a).