To provide a cast joined body and its producing method with which wear resistance, heat resistance, rigidity and also, corrosion resistance can be improved while utilizing the light-weight of a Mg alloy.

The cast joined body is composed by joining a member to be cast composed of a Fe alloy, a Ni alloy or a Cu alloy and a cast member composed of a Mg alloy by interposing an electrical insulating layer on the interface between the both members. This producing method is performed as the followings. (1) After forming a film composed of material forming the electrical insulating layer on a part of or on the whole surface of the member to be cast by reacting with molten Mg alloy, the member to be cast, this is cast with the Mg alloy. (2) After forming the electrical insulting layer composed of an oxide on the surface by heating the member to be cast, the member to be cast, is cast with the Mg alloy. As the other embodiment, in the method for cast the member to be cast, which is composed of an Al alloy or an Al base composite material with the cast member composed of the Mg alloy, after forming the film composed of material reacting with the molten Mg alloy on a part of or on the whole surface of the member to be cast, the member to be cast, this is cast with the Mg alloy.