To provide a folding chair capable of quickly expanding its seat part and the folding smoothly and exactly by having a general purpose air cylinder built-in.

On this chair, a seat part 4 and the first weight 8A, which energizes to the direction of folding of the seat part 4 by increasing in weight of the back side of the first weight 8A heavier than that of front side with a rotation axis 6 as the center, are placed. In the internal hollow member 5, an air cylinder 11, which has a rod 12 that uses the air in the internal hollow member 5 and the first weight 8A to work the energizing power to the direction of folding the seat part 4, is built in, and a rack gear 8a is set on the tip side of rod 12. Then a gear 6a, which meshes with the rack gear 8a, is formed on at least one peripheral end side of rotation axis 6, and the second weight 8B, which adjusts the elasticity of rod 12 of air cylinder 11, is placed on the upward rack gear 8a.