To retain the contact face with a human body to form a smooth curve by connecting the rear face side stretching over the bend of a cushion body by a supporting belt-form member and pressing the bend to the surface side by the tension of the support member when the cushion body is bent.

A support member 27 with a specified tension is provided between the upper part 16a and the lower part 16b of a back seat to connect them and stretch over a bend 29 of a cushion body 25. And hence, a pressing force toward the surface (arrow mark direction) is generated at both bends 29 by the support member 27 of which tension has been increased when the upper and lower parts 16a, 16b of the back seat have been bent, to prevent the bend 29 from being extremely bent. And hence, the bend 29 is formed to make a smoothly curved face to improve the sitting sense. In such a chair, when a sitting person leans back, the chair works so as to support his back while inclining the upper part 16a of the back seat forward.