PURPOSE: To provide a moistureproof composite plastic board having heat insulating function and moistureproof function and easy to handle by laminating a moistureproof sheet to a hollow plate-shaped object made of plastic.

CONSTITUTION: A hollow plate-shaped object 1 made of plastic and a moisture proof sheet 2 are laminated and boned. As the hollow plate-shaped object 1 made of plastic, for example, a hollow laminated sheet made of plastic such as Prapal (R) [phonetic] (made by Japan Petrochemical Co.). By this method, a lightweight moistureproof composite plastic board easy to handle, having moistureproof properties, antifungal properties and dewing resistance, enhanced in plane compression strength and impact resistance and useful as a building material or a box making material is obtained.