PURPOSE: To manufacture the titled alloy plate with superior surface accuracy by annealing and cold rolling a plate cast continuously from a molten Al alloy contg. Mg essentially, Ti and B selectively and Fe, Mn, Si and Cr each restricted to a specified amount or below.

CONSTITUTION: A molten Al alloy contg., by weight, 2.0W3.5% Mg as an essential alloying element, 0.004W0.08% Ti and 0.0005W0.01% B as selective alloying elements and Fe, Mn, Si and Cr each restricted to ≤0.1% is used. The molten alloy is passed through a filter applying electrochemical absorption phenomenon to remove nonmetallic oxides in the molten alloy, and it is cast into a continuously cast plate having 4W10mm thickness. The cast plate is quenched, optionally cold rolled at ≥25% draft, annealed at 400W550°C for 1W48hr, and further cold rolled at ≥30% draft.