PURPOSE: To obtain the pure magnesium substrate which has no intermetallic compd. and grain boundaries and has a high memory density by subjecting a planar casting ingot of magnesium consisting of the single crystal formed by casting the magnesium having specific purity by using a heated casting mold to rolling.

CONSTITUTION: The melt of the high-purity magnesium of 99.98% is supplied into the hollow heated casting mold heated to ≥650°C solidification temp. in an inert gaseous atmosphere and the cooling of the casting ingot is executed only on the outside of the outlet of the casting mold. A slab consisting of the unidirectionally solidified structure of the magnesium is then obtd. and such unidirectionally solidified structure is grown finally to the single crystal by the competitive growth of the crystals on progression of the casting. The plate which causes work hardening, has high rigidity and has no intermetallic compd. and grain boundaries on the surface is obtd. if such thin single crystal plate of the magnesium is worked at a low temp. of ≤100°C. The formation of the magnesium substrate for optical disk which is extremely light and thin is possible in this way.