A sheet of silicate glass having a thickness of 2 mm and composed mainly of SiO2, and containing Al2O3, B2O3, Na2O, F, etc., is immersed in a molten salt comprising a mixture of 50 mol % of silver nitrate and 50 mol % of sodium nitrate. Na ions in the surface of the glass are eluted, diffusing Ag ions in the molten salt into the glass. When a laser beam is applied to the glass substrate thus formed, the glass substrate is evaporated or ablated progressively from its surface. The glass substrate is processed to a smooth finish without causing cracking or breakage.