PURPOSE: To manufacture a high-strength Mg alloy for plastic working excellent in plastic workability such a forgeability, by subjecting a molten Mg containing specific amounts of A$, Zn, and Mn to continuous casting under specific conditions or by subjecting an ingot of the Mg alloy to hot plastic working under specific conditions.

CONSTITUTION: A molten Mg alloy containing, by weight, 1.0W10.0% Al, 0.1W2.0% Zn, and 0.05W1.0% Mn is casted continuously and solidified at ≥25°C/sec solidification rate to be formed into a continuously cast billet, so that an Mg alloy in which the size of the grains of the secondary phase consisting of one or two intermetallic compounds among Mg-Zn, Al-Mn, Mg-Al- Zn, Al-Mn-Fe, Mg-Si, etc., is regulated to ≤30μm and the greater part of the metallic structure has fine a equi-axed crystal of ≤200μm average grain size is manufactured, or, an Mg alloy prepared by casting a molten Mg alloy with the above-mentioned composition and subjecting the resulting ingot to hot plastic working at 220W450°C at ≥25% draft is manufactured. In this way, the high- strength Mg alloys respectively excellent in plastic workability such as forgeability can be obtained.