To provide a stiffness controlling device for body contact part of a chair, which excels in maneuverability of controlling stiffness of the body contact part and has a small deformation amount of the body contact part surface, stability and good fitting touch.

To control stiffness of a body contact part by perforating an opening 16 on a shell 10 which supports a cushion body of a chair and installing a pair of right and left guide rails 14 to the both sides of the shell 10. Stiffness of the body contact part is controlled by varying the deformative amount of the cushion body and by installing a blocking plate 12 which can be shifted, be guided by the guide rails 14, and block the whole or a part of the opening 16.