A movable rail body is fastened to a movable body and has a first space. Movable rail bearings are disposed in the first space at diagonally facing corners of the movable rail body. A stationary rail body is fastened to a stationary support and has a second space. Stationary rail bearings are disposed in the second space at diagonally facing corners of the stationary rail body. A slider has an upper end in rolling contact with the movable rail bearings, and a lower end in rolling contact with the stationary rail bearings. An angle between a center line parallel to the movable body and a connection line connecting centers of the movable rail bearings ranges from 40 degrees to 50 degrees. An angle between the center line and a connection line connecting centers of the stationary rail bearings ranges from 130 degrees to 140 degrees.