To solve the problem that a bathroom wall material having high functionality cannot be manufactured at a low cost.

A heat-insulating material 213 is pressed in between steel plates 211 and 212 and the heat-insulating material 213 in the proximate portion to the steel plate 211 and the steel plate 212 is softened by hotpressing the material 213 in the thickness direction. Consequently, bubbles are compressed and the heat-insulating material 213 is stuck fast to the steel materials 211 and 213. Accordingly, rigidity at the proximate portion of the steel plates 211 and 213 is increased at a time when the material 213 is cooled and solidified, and a screw can be screwed firmly. Since the surface of the material 213 is welded to the steel plates 211 and 213, a working process for bonding with each steel plate is not required.