To reduce weight, and strengthen flexural strength by layering a cement layer or a gypsum layer in an impregnation layer-formed state on the obverse/reverse of an orientative strand board by preapplying dimension stabilizing treatment by heating steam treatment.

A ligneous strand is molded into a plate shape on an orientative strand board Wa through a phenol resin, MDI, a urea resin and a melamine resin, a specific gravity is reduced, and higher flexural rigidity is imparted than the other fiber board and a particle board. However, even the orientative strand board Wa has inconvenience such as a using place is limited since a rate of dimensional change is large by absorption/release of moisture similarly to the particle board. Then, a material applied with dimension stabilizing treatment on the orientative strand board Wa by heating steam treatment, is used as a core material of an architectural composite plate A, and a cement layer or a gypsum layer G is layered on the obverse/reverse to thus obtain a fire resistant architectural composite material having strong flexural strength and high dimensional stability.