A magnesium alloy part is inserted into a mold, a resin composition is injected and joined to the part, and a composite is obtained. A part having, formed thereon, a surface layer of a metal oxide, a metal carbonate, or a metal phosphate in use of a usual conversion treatment or a modification method thereof can be used for the magnesium alloy plate 1. The surface that has a larger amount of crystal-like objects of a nanolevel on the surface layer composed of the metal oxide, metal carbonate, or metal phosphate has a higher level of hardness, microscopic roughness, and good injection joining force, and these parameters can be controlled by a conversion treatment method. A resin composition 4, containing PBT or PPS as the main component, is used as the resin composition part.