In a method of removing a useless film formed along a circumferential portion of a substrate, so as to provide a removed portion, a cover member is covered over the substrate to supply a solvent to the useless portion through solvent supply holes. The circumferential portion which has the removed portion and a non-removed portion serves to provide an identification code or the like for identifying the substrate. The solvent supply holes are formed in a solvent guide member which is exchangeably attached to a peripheral portion of the cover member. The substrate may be either a mask blank or a mask that has the identification pattern formed by removing the useless film by the above-mentioned method.