PURPOSE: To manufacture an Mg base alloy excellent in corrosion resistance as well as hardness and strength by superrapidly cooling the molten metal of an alloy having a specified compsn. essentially consisting of Mg.

CONSTITUTION: The molten metal 3 of an Mg base alloy having a compsn. expressed by general formulae [I] to [IV] [in the formulae, X denotes ≥ two kinds among Cu, Ni, Sn and Zn, M denotes one or ≥ two kinds among Al, Si and Ca and Ln denotes one or ≥ two kinds among Y, La, Ce, Nd and Sm or misch metal, and as for the value of (a) to (e), by atomic %, 40≤a≤95, 5≤b≤60, 1≤c≤35, 1≤d≤25 and 3≤e≤25 are regulated] is poured into a silica tube 1, is dropped from a small opening 5 at the lower part on a rapidly rotating cooling roll 2 made of copper and is solidified into the shape of a thin sheet 4. The Mg base alloy constituted of a crystalline phase of which various intermetallic compounds are finely dispersed into an Mg matrix and an amorphous phase contg. Mg and other incorporated elements and excellent in hardness, strength and corrosion resistance can be manufactured.