PURPOSE: To reinforce the product without increasing its thickness and to reduce the material cost and the working cost of the product by interposing a reinforcing mat composed of a glass fiber between a upper mold and a lower mold, injecting a resin between the upper mold and the lower mold, and molding the product.

CONSTITUTION: A lower mold 2 of a metallic mold to mold wash bowl 1a is formed into a shape to mold its surface side, a flange molding part 4 is provided for the periphery of a bowl molding part 3 of the lower mold 2, and an exhaust port molding part 5 is provided for the neighborhood of the center of the lower mold 2. A reinforcing mat 6 composed of the glass fiber is arranged at the center of the bowl molding part 3 of the lower mold 2 as a position corresponding to a bowl bottom part for which the reinforcing of the wash bowl 1a is necessary. The reinforcing mat 6 composed of the glass fiber is, for example, a chipped strand mat at manufactured by cutting the strand of the glass fiber into approximately 50mm, laminating the cutted pieces in a non-oriented manner so that a uniform thickness can be obtained, binding the laminated pieces with a binding material, etc., and cutting the bound pieces into a piece having a necessary size and a necessary shape. When the bottom surface of the wash bowl 1a is to be reinforced, the reinforcing mat is cut into an approximately 30-40cm square and placed in the neighborhood of the exhaust port molding part 5. The upper mold (not shown in figure) is put over the lower mold 2, the upper mold and the lower mold 2 are closed, the resin such as an acrylic resin is injected between the upper mold and the lower mold 2, the resin in heated, pressurized, and hardened, and the wash bowl 1a molded after the hardening is taken out.