Abstract: A chair is provided which includes an inner member for supporting the body of a person and an outer member for supporting the right and left edges of the inner member. The inner member is made of a synthetic resin and formed into a plate so that the inner member is elastically deformed under the weight of the person. The inner member is attached to the outer member with its side portions supported. Thus, behind the inner member is a space for allowing the inner member to be deformed. In order to be applied to the seat of a chair, preferably, the outer member includes a metal base and a pair of auxiliary supporting elements to be fitted on the metal base. The inner member and the auxiliary supporting elements are fixed to each other in a non-releasable manner by an engaging assembly including engagement nails and engagement holes.