The surface of a glass substrate withdrawn from a chemically strengthening treatment liquid is processed with a heated acid (for example, a thermal concentrated sulphuric acid at a temperature of over 100.degree. C., or a water soluble organic solvent such as glycerol or polyethylene glycol). Consequently, precipitated fused salt can be effectively removed without damaging the glass substrate and the change in quality of the glass surface can be also prevented at a high level.