Abstract: A direct-acting actuator for an industrial robot has an elongate ball screw or feed screw which is attached or detached quickly and safely in a narrow working space. In attaching the ball screw, upper and lower end portions of the ball screw, diagonally inserted in the inside space of a column (10), are respectively fitted into holes (12a, 33a) of a column top plate (12) and a plate member (33) of a slider (30) through a passage (12b), formed in the top plate and opening in a slider-side end face of the same plate, and a passage (33b), formed in the plate member and opening in the distal end face of the same member. Then, the lower end of the ball screw is coupled to a reduction gear (52), and a bearing unit (22) mounted on the upper end of the ball screw and a ball nut (34) threadedly engaged with the ball screw are respectively fitted into the holes (12a, 32a) so that the bearing unit and the ball nut are fixed to the top plate and the plate member, respectively.