To enable a stool lid to open and close, engaging a hook part with the rear edge part of the stool lid, even if the width of the gap between the rear edge part of the stool lid and the main part is shorter than the longitudinal length of the fold-back part of the hook part.

The part between the top part 18a of a hook part 18 and the last part 18b is called a fold-back part 18c. The part between the top part of the base part and the fold-back part 18c is called a support part 18d. The whole curved part 18d1 of the support part and the part in front of the curved part 18d1 of the support part is called the wide-width plate part 18e. The part between the top part 18a and the start point of the wide-width plate part 18e of the hook part 18 is called a narrow width plate part 18f which is approximately a third of the wide-width plate part 18e.