PURPOSE: To obtain a composite material, which is not expensive and at the same time can be easily cut with a cutter knife or the like, is easily worked, lightweight and excellent in heat insulating properties and in lightscreening properties by a method wherein a resin layer is formed on the surface of corrugated fiberboard by adhering resin to said surface.

CONSTITUTION: Corrugated fiberboard is immersed in molten state fiber reinforced thermosetting resin (FRP) such as unsaturated polyester or the like and immediately after that pulled up from the molten resin in order to harden the FRP adhered to the surface of the corrugated board. Since a thin layer of FRP is formed on the surface of the corrugated board and the paper at the surface of the corrugated board is partially impregnated with the resin and hardened, composite material of hardened paper quality can be obtained. The obtained composite material can be used for molding a water storage tank 1, for example. Concretely, the outer frame and inner frame of a mold for molding the tank consist of the composite material. After a molten resin such as FRP or the like are cast between the frames, the resin is hardened for molding the water storage tank 1.