PURPOSE: To form a strong mask which is not corroded by molten salt even if the mask is subjected to ion exchange treatment for a long time by forming recessed parts on the surface of a glass base plate and allowing the air layer held in the recessed parts to contact with molten salt as the mask.

CONSTITUTION: Recessed parts 5 including circuit part are formed on one of the surface of a multicomponent glass base plate 3 by sputter-etching, etc. With air layers 6 held in the recessed parts 5, the glass base plate 3 is dipped in molten salt 2 with the recessed parts facing downward. Under this condition, univalent metal ions contained in the molten salt 2 are exchanged with univalent metal ions contained in the glass base plate 3. By the ion exchange, refractivity index distribution exhibiting lens effect is generated in the multicomponent glass base plate 3. In this stage, the air layer 6 serves as mask against the contact of the glass base plate 3 with the molten salt 2. The mask of the air layer 6 has superior chemical resistance being durable to long time ion exchange treatment.