To enable a process to make a product lightweight as thin as possible irrespective its high flexural rigidity and strength compatible with a surface material, and possible to be produced relatively inexpensively.

A core material 7 placed between two opposite surface material is formed to have an uneven cross section by bending a thin steel plate. The first-fourth contact surfaces 71, 73, 75, 76 which are contacted alternately with the inner surfaces of two surface materials and the first-third support surfaces 72, 74, 76 which connect these contact surfaces are provided continuously. The first contact surface 71 is contacted with the inner surface of one surface material, the first support surface 72 is bent from the first contact surface to extend toward the other surface material. The second contact surface 73 is bent from the first support surface to be contacted with the inner surface of the other surface material, and the second support surface 74 is bent from the second contact surface to extend toward one surface material.