A heat-resistant magnesium alloy being characterized in that: it is a heat-resistant magnesium alloy that includes: magnesium (Mg) making the major component; aluminum (Al) in an amount of from 1.34 or more to 1.63 or less by an atomic ratio of Al with respect to Ca (i.e., Al/Ca); calcium (Ca) in an amount of from 1.235 atomic % or more to 2.470 atomic % or less; and manganese (Mn) in an amount of from 0.13 atomic % or more to 0.27 atomic % or less; the balance comprising Mg and inevitable impurities; when the entirety is taken as 100 atomic %; and it 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; said 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; and a relative angle between a normal vector to the hexagonal-system basal plane of said Mg crystalline grains and another normal vector to the hexagonal-system basal plane of said 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, and wherein the heat-resistant magnesium alloy does not include a magnesium-alloy composition consisting of 2.75 atomic % Al, 1.85 atomic % Ca, 0.23 atomic % Mn, and the balance being Mg. it is a heat-resistant magnesium alloy that includes: magnesium (Mg) making the major component; aluminum (Al) in an amount of from 1.34 or more to 1.63 or less by an atomic ratio of Al with respect to Ca (i.e., Al/Ca); calcium (Ca) in an amount of from 1.235 atomic % or more to 2.470 atomic % or less; and manganese (Mn) in an amount of from 0.13 atomic % or more to 0.27 atomic % or less; the balance comprising Mg and inevitable impurities; when the entirety is taken as 100 atomic %; and it 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; said 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; and a relative angle between a normal vector to the hexagonal-system basal plane of said Mg crystalline grains and another normal vector to the hexagonal-system basal plane of said 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, and wherein the heat-resistant magnesium alloy does not include a magnesium-alloy composition consisting of 2.75 atomic % Al, 1.85 atomic % Ca, 0.23 atomic % Mn, and the balance being Mg. magnesium (Mg) making the major component; aluminum (Al) in an amount of from 1.34 or more to 1.63 or less by an atomic ratio of Al with respect to Ca (i.e., Al/Ca); calcium (Ca) in an amount of from 1.235 atomic % or more to 2.470 atomic % or less; and manganese (Mn) in an amount of from 0.13 atomic % or more to 0.27 atomic % or less; the balance comprising Mg and inevitable impurities; when the entirety is taken as 100 atomic %; and it 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; Mg crystalline grains; and grain-boundary crystallized substances being crystallized out in a grain boundary between the Mg crystalline particles; said 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; and a relative angle between a normal vector to the hexagonal-system basal plane of said Mg crystalline grains and another normal vector to the hexagonal-system basal plane of said 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, and wherein the heat-resistant magnesium alloy does not include a magnesium-alloy composition consisting of 2.75 atomic % Al, 1.85 atomic % Ca, 0.23 atomic % Mn, and the balance being Mg.