In joining a magnesium alloy material 1 (first material) and a steel material (second material), a zinc-plated steel plate 2 plated with zinc (metal C) is used as a steel material, Al (metal D) is added to the magnesium alloy material 1. Next, eutectic melting of Mg and Zn is caused so as to remove a product produced by the eutectic melting with an oxide film 1f and impurities from a joint interface. Moreover, an Al—Mg system intermetallic compound such as Al3Mg2 and an Fe—Al system intermetallic compound such as FeAl3 are produced, whereby regenerated surfaces of both materials 1 and 2 are joined via a compound layer 3 containing those intermetallic compounds.