To provide excellent fire resistance by providing a thin-walled rust proof steel plate on both surfaces or a single surface and interposing a fire resisting material mainly composed of a foamed material or plaster between both surfaces or on the other surface.

A thin-walled steel plate 2 is provided on both surfaces where a foamed material having proper magnification and density is interposed between the steel plates 2, thereby molding a board having a specified thickness. Then, a thin-walled rust proof steel plate 22 is provided on a single surface while a fire resistant material 5 mainly composed of plaster, such as coal waste or shirasu is pasted on the other surface, thereby molding a board 4 having a specified thickness. In the case when these boards 1 and 4 are used as a concrete form in building work or civil engineering work, onsite work, such as drilling or cutting can be executed relatively easily. It is, therefore, possible to increase the number of diversion dramatically and provide light weight and durability performance.