PURPOSE: To minimize the quantity of a defect exposed on the surface of a rolling material after working by setting optimumly the ratio of a second plate thickness of a target after the finish working and a first plate thickness before the working.

CONSTITUTION: A magnetic disk use blank 1 obtained by bringing an ingot of an Al alloy to rolling working has such a tendency as the distribution density of a defect decreases gradually in the internal direction from the surface in the plate thickness direction and increases again as it approaches the center part. By setting plate thickness T0 of the magnetic disk use blank material 1 of a stock to a prescribed value, based on a value of plate thickness T1 after finish-working so that an area in which said defect becomes minimum goes to the surface of a substrate 1a of the plate thickness T1 after finish-working, the quantity of the defect which is exposed on the surface of the substrate 1a and may be the cause of deterioration of the yield can be minimized.