To obtain a low cost and the feel of a material by improving incombustibility, working performance, and decorative properties by a method wherein a synthetic resin layer is formed on the surface of an incombustible material, and a gel coat resin layer or a coating layer is formed on a surface of the synthetic resin layer.

After applying a preliminarily prepared grain toned gel coat resin material to molding surfaces of a front mold 3 and a rear mold 4 of a molding tool with a spray gun 10, that is cured by heating to form a gel coat resin layer 6. Then, a synthetic resin material for bonding is applied onto a surface of the gel coat resin layer 6 with a spray gun 11 to form a synthetic resin layer 8. Besides, a primer treating material 13 is prepared, and after applying it to a front surface 5 and a rear surface 7 of an asbestos slate board with the spray gun, that is cured to form an incombustible material layer 14. While the primer treating material 13 on the surfaces of the synthetic resin layer 8 and the incombustible material layer 14 is uncured, the incombustible layer 14 is placed on the synthetic resin layer 8, and then a rear mold 4 is applied from above the incombustible material layer 14, the front mold 3 and the rear mold 4 are clamped with a jack, and the primer treating material 13 is cured.