PURPOSE: To strongly bond unsaturated polyester resin moldings by a procedure in which a specific selectively adsorptive substance is adhered to an unsaturated polyester resin, the polyester is molded and hardened, the adsorptive substance is removed, and then the unhardened polyester resins are doubled and hardened.

CONSTITUTION: An unsaturated polyester resin containing a vinyl monomer (a), e.g., of styrene, etc., and an unsaturated alkyd resin (b) is impregnated into a reinforcing material and then laminated on the surface of molds. Then, a selectively adsorptive substance capable of adsorbing a component (a), e.g., silicone rubber, urethane resin, etc., but incapable of adsorbing a component (b) is adhered to the place to be secondarily bonded of the reinforcing material, molded, and then hardened. Then, the adsorptive substance is removed out and then the hardened resin so deprived of the adsorptive substance is superposed with a reinforcing material impregnated with unhardened unsaturated polyester resin and then hardened.