To provide a chair which can be comfortably seated without requiring high technical capabilities or labor hours for manufacturing.

A middle part A6 in the chair A1 is provided with: a plurality of vertical crosspiece areas A61 which have wavy shapes, continuously projecting and sinking in a plate thickness direction with a constant cycle in a longitudinal direction; and a plurality of horizontal crosspiece areas A62 constituted by continuing the plurality of vertical crosspiece areas A61, each other. The phase of the continuous wavy shape of the adjacent vertical crosspiece areas A61 is shifted. The chair has a cross-section shape which can be deformed in a direction in which the cycle for projecting and sinking in the plate thickness direction is increased when the load is applied to the plate material A31 in the thickness direction.