To prevent a projection defect due to residual magnetic polisher slurry or a recess defect due to magnetic polishing slurry carried in a succeeding step, even when a substrate is subjected to local polishing using magnetic polishing slurry.

After the surface of a mask blank substrate 1 is polished by using magnetic polishing slurry 2 prepared by incorporating polishing abrasive grains into a magnetic fluid 21 containing iron, the surface of the mask blank substrate 1 is cleaned by using a cleaning liquid 6 containing hydrochloric acid.