To prevent occurrence of a symptom of staggering or the like on the occasion of standing up after sitting straight for long time.

A seat and a seat support outer frame base are integrated by fitting a seat support in the seat support outer frame base 7. Level adjusting-fixing round half holes 6 are opened in six places in the outside of the seat support, and the level adjusting-fixing round half holes 6 are opened in six places in the inside of the outer frame base, while level adjusting-fixing round holes 6 are opened at six places in the seat support and the outer frame base respectively. A hard reinforced plastic ball 2 and a hard reinforced plastic spring 8 are installed in the uppermost portion round half holes of the level adjusting-fixing round half holes, in a gap between the seat support outer uppermost portion and the outer frame base inner uppermost portion, and then a holding-reinforcing reinforced plastic plate is attached to the outside of the outer frame base and tightened by a stainless steel screw. Thereby positions of the seat support and the outer frame base level adjusting-fixing round half hole and of the seat support and the outer frame base level adjusting-fixing round hole are set in a line horizontally and thus the level of the seat support can be adjusted and fixed. The height is fixed by inserting a seat support adjusting-fixing T-type handle 1 (round hard metal rod) being an attachment into the seat support and the outer frame base level adjusting-fixing round hole 3.