To prevent generation of permanent deformation in a moving or guiding track by a large load of a ball.

A seat fixable to a selectable vertical direction position particularly an automobile seat comprises a guide rail 10 provided in each seat major side and fixed to a bottom of a vehicle and a moving rail 11 movable relating to the guide rail 10, and the moving rail 11 is supported onto a roller 16 of a lower part moving track 18 and to a guide track of the guide rail 10 through a ball 23. In a front/rear part range of at least one guide rail 10, respectively a pair of the balls 23 are provided, and, between these balls 23, a molded piece having a diameter slightly smaller than a diameter of the ball 23, for instance, a roller 24, is arranged.