PURPOSE: To lower water absorptivity by compounding a specific ratio of a powdery thermoplastic resin with a cement admixture composed of cement, silica, pulp fibers, pigment, etc.

CONSTITUTION: The powdery thermoplastic resin is compounded at 3-7wt.% with the cement admixture consisting of the cement, silica, pulp fibers for reinforcement, pigment and other fillers. PVC, PE, acrylic resin, etc., are usable as the thermoplastic resin. The effect of lowering the water absorptivity of the board material is insufficient if the compounding ratio thereof is <3%. The above-mentioned effect is not so much improved even if the ratio exceeds 7%. The resin is melted to flow by heating at the time of curing so as to flow into the gaps included in the board material when the board material is molded of the above-mentioned compd. and is cured in an autoclave. The resin solidifies in the gaps after the curing. The volume of the gaps is thereby decreased and the closed cells are formed simultaneously in the board material.