To solve the problems that sputter flies into the screw holes of nuts for adjusting a height, working performance is poor and a cost is high because an upper bottom plate is welded to a lower bottom plate.

Fitting grooves are formed at the upper bottom plate to fit the lower bottom plate to the fitting grooves. The upper bottom plate and the lower bottom plate are connected and fixed by one or at least two combinations of a rivet, a tapping screw, a bolt and a nut, spot welding and an adhesive. The lower bottom plate is fitted into the fitting grooves of the upper bottom plate and both the bottom plates are connected and fixed by one or at least two combinations of the rivet, the tapping screw, the bolt and the nut, spot welding and the adhesive.