An actuator has a frame and a slider body which are both made of extruded shapes of light-metal alloy. A first rail member is fastened to the frame by means of a simple construction. An upper sealing member is placed between the frame and the slider body. With the actuator constructed as stated earlier, the frame is an elongated extrusion of U-shape in transverse section whose side walls have first recesses of dovetail configuration to fit over first rail members and flanges. A slider has therein the slider body having second recesses of rectangular shape to fit over second rail members. There is further provided a driving unit to force the slider with respect to the frame. The first rail members each have the wedged configuration including tapered contours and dents, while the second rail members each have the rectangular shape.