To reduce the number of components and to manufacture simply at a low cost.

In a chair structural body such as seat, backrest, provided with a cushion 3, a facing cloth 4 spread over the cushion 3, and structure plate material made of a synthetic resin to support the cushion 3 and facing cloth 4, with the cushion 3 and facing cloth 4 housed in an injection mold 10 for the structure plate material 2 made of a synthetic resin, and a fused resin material 11 is injected to form the structure plate material by low-pressure injection molding. The structure plate material, cushion 3, and facing cloth 4 are integrated simultaneously with the formation of the structure plate material to integrally form the cushion 3 and facing cloth 4 with the structure plate material.