A heat-resistant magnesium alloy containing, based on the total weight of the alloy, 4.5-10 wt. % of aluminum, 0.1-3 wt. % of calcium, 1-3 wt. % of a rare earth element and 0.2-1 wt. % of manganese and having a composition that the contents of aluminum, calcium and the rare earth element satisfy the relationship of the following expression (1): 1.66+1.33 Ca+0.37 RE.ltoreq.Al.ltoreq.2.77+1.33 Ca+0.74 RE wherein Ca, RE and Al represent the weight percentages of Ca, at least one rare earth element and aluminum contained in the alloy, respectively, in the relationship.