A sliding device is provided in which a ball-nut screw system is disposed in a relation lying out of a guide rail and extending by the side of and in parallel juxtaposition with the guide rail, thereby making it possible to reduce the sliding device in the overall height. The ball-nut lead screw system may be easily mounted and/or demounted from the sliding device and easily made varied depending on design considerations. The sliding device includes the lead screw carried for rotation by bracket bearings in a way lying out of a guide rail in a way extending by the side of and in parallel juxtaposition with the guide rail, and a ball nut mounted to a slide table. The lead screw is actuated by a motor carried in a bracket bearing.