To shorten the period of works by forming a building plate of surface and reverse layers having wood fiber bonded by a liquefied synthetic resin binder, and a core layer having a mixture of an inorganic starting material and a reinforcing fiber bonded by a liquefied synthetic resin binder.

A mixture containing 92-85wt.% of a wood fiber and 8-15wt.% of a liquefied synthetic resin binder formed of urethane resin or melamine resin is sprayed onto a die plate to form a surface layer mat B1. A mixture containing 18-45wt.% of an inorganic starting material formed of shirasu burn or pearlite, 45-72wt.% of a reinforcing fiber formed of polyester fiber or polyethylene fiber, and 8-15wt.% of the liquefied resin binder is sprayed on the form plate to form a core layer mat B2. Further, a mixture of wood fiber and the liquefied synthetic resin binder is sprayed on the core layer mat B2 to form a reverse layer mat B3, and each layer is heated and compacted to form a composite building plate B. Thus, the manufacturing process can be significantly shortened.