Abstract: A heat-resistant magnesium alloy according to the present invention has Mg as the major component, includes Al, Ca and Mg, and has a metallic structure that comprises Mg crystalline grains and grain-boundary crystallized substances being crystallized out in a grain boundary between the Mg crystalline particles. The grain-boundary crystallized substances comprise a mixed crystal phase of a Laves-phase compound with a type-“C14” crystalline structure, and another Laves-phase compound with a type-“C36” crystalline structure. Moreover, a relative angle between a normal vector to the hexagonal-system basal plane of the Mg crystalline grains and another normal vector to the hexagonal-system basal plane of the grain-boundary crystallized substances is from 88 deg. to 92 deg. at an interface between the Mg crystalline grains and the grain-boundary crystallized substances at least.