An actuator has a hollow track rail 6 having a slit 8 extending in an axial direction thereof, a movable block 7 disposed inside the track rail 6 to be movable along the track rail, and a ball screw mechanism for moving the movable block 7 along the axial direction of the track rail 6. The track rail has, in a section perpendicular to the axial direction of the track rail, a guide portion 9 for guiding movement of the movable member 7 and an extension 11 extending from the guide portion 9 so as to cover the movable member, and a width W1 of the slit 8 of the track rail formed between the opposed extensions 11 is narrower than a width W2 of the movable member. Accordingly, the actuator can provide a high rigidity in spite of compact outer dimension.