A guide rail has two roller running surfaces and two ball running grooves. A movable block has a horizontal member facing the upper surface of the guide rail and wing members respectively facing the side surfaces of the guide rail. The horizontal member has on its under surface two roller running counter-surfaces corresponding to the two roller running surfaces of the guide rail. Each of the wing members has on its inner surface two ball running counter-grooves corresponding to the two ball running grooves of the guide rail. Two trains of rollers are arranged between the two roller running surfaces and the two roller running counter-surfaces, respectively, so as to be rollable. Two trains of balls are arranged between the two ball running grooves and the two ball running counter-grooves, in each of gaps between the side surfaces of the guide rail and the inner surfaces of the wing members, so as to be rollable.