To provide a comfortable chair by requiring neither high technical capabilities nor labor for manufacture.

The chair includes a plate A31 including a periphery A5 supported by a chair body A2 and a middle part A6 positioned inside the periphery. A specific place of the middle part A6 is structured such that a plurality of beams A61 deflectable in a thickness direction of the plate are disposed adjacently and the beams A61 are connected intermittently by a connection member A62. The respective beams A61 include a groove A63 open to a back side and have a sectional shape deformable in a direction where the groove A63 is enlarged and opened when a load is applied in a thickness direction of the plate A31.