A refractory magnesium alloy includes magnesium as a principal ingredient, and an element having a radius 9 14% larger than a magnesium atom and a maximum concentration of 2 mass % or larger in a solid solution with magnesium is mixed in an amount not exceeding a maximum amount that can be homogeneously mixed in the solid solution with magnesium, whereby internal strength of grains thereof is enhanced. Alternatively, gadolinium with a content thereof ranging from 0.5 to 3.8 mass % is added, so that remaining part other than the gadolinium is composed of the magnesium and unavoidable impurities. This magnesium alloy serves to inhibit decrease in proof stress and creep deformation, especially primary creep deformation when used at high temperatures, typically at 200.degree. C. The magnesium alloy may be employed for a structural material for a vehicle, so that a lightweight and heat-resistant structural material can be obtained.