PURPOSE: To provide a magnesium alloy excellent in heat resistance, corrosion resistance and castability.

CONSTITUTION: The objective magnesium alloy contains, by weight, 1.0 to 6.0% Zn, 0.1 to 2.0% R.E., 0.1 to 2.0% Zr, 0.1 to 3.0% Si and 0.1 to 6.0% Al, and the balance Mg with inevitable impurities. By Al and Zn, its castability can be improved as well as its room temp. strength can be improved. Moreover, by adding R.E. besides Al and Zn, Mg-Al-Zn-R.E crystallized products are crystallized out on the grain boundaries to improve its high temp. strength. In addition, the content of R.E. is reduced in a range in which its high temp. strength can be maintained, by that, its castability is made excellent, its tensile strength at a room temp. is made high and its high temp. properties and creep properties are made improved. Furthermore, R.E. forms a R.E. rich protective film in the initial period of corrosion to improve its corrosion resistance. By the incorporation of Zr, its room temp. strength and high temp. strength improve without deteriorating its castability, and, by the incorporation of Si, its creep resistance improves.