PURPOSE: To provide an automatic adjusting mechanism for the inclination reaction force of a back rest of a chair, when senses the weight of a seated person at the time of inclination of a back rest, and can adjust automatically force for opposing reaction force generated by the inclination generated by a spring for supporting elastically the back rest by the weight of the seated person.

CONSTITUTION: In the chair in which the seat surface 9 and a back rest 4 are supported by a base member 2 attached to a leg pillar 1, etc., and also, the inclination of the back rest 4 is supported elastically by the resiliency of a spring, this chair is constituted so that the side for receiving reaction force by the inclination force of the spring is supported directly or indirectly by the seat surface 9, and a load by the weight of a seated person, etc., applied to its seat surface 9 is allowed to work on the spring as the force for receiving reaction force of the spring.