The present invention is useful for achieving a reduction in weight and for attaining increased strength in not only electronic devices and domestic electric devices but also various parts and structures. As a pretreatment, a rib (3) is dipped in an aqueous solution of ammonia, hydrazine, a hydrazine derivative, or a water-soluble amine compound. A metal frame (2) is inserted into an injection mold for forming ribs (3) by injection molding. A thermoplastic resin composition is injected to the surface of the metal frame (2) by injection molding to form ribs (3). In the housing of a casing cover (1) thus formed, the metal frame (2) and the ribs (3) made of the thermoplastic resin composition are integrally bonded together. Thus, the housing improves strength and external appearance. Moreover, a complicated configuration and structure can be formed in the housing.