A--Si containing magnesium alloy for casting with a melt thereof with a cast structure having eutectic compounds of Mg.sub.2 Si produced to improve creep strength of a cast product, preferably a Si-containing magnesium alloy of a Mg--Al--Zn system having either 0.01% to 2.0% or 6 to 12% of Zn and 6 to 12% of Al, is disclosed with the improvement in that the alloy contains 0.3 to 1.5% by weight of Si in combination with 0.005 to 0.2% of Sr added to effect refinement of the eutectic compounds to thereby reduce hot cracking and improve mechanical properties of the cast product, while the improved creep strength is preserved.