PURPOSE: To manufacture an index gradient-type lens having excellent lens characteristics and stable qualities, in high efficiency, by carrying out the ion exchange of glass matrix with a molten salt while replacing a part of the molten salt with fresh molten salt.

CONSTITUTION: The ion exchange bath 1 containing molten salt 20 such as potassium nitrate, sodium nitrate, etc. is heated externally with the heater 2, and the temperature of the molten salt 20 is controlled at a constant level with a thermocouple 3. A number of thin rods 15 of the matrix glass for lens to be subjected to ion-exchange treatment are suspended and immersed in the molten salt 20. The freshly melted molten salt is heated in the auxiliary tank 8 with the surrounding heater 9, and controlled accurately at a constant temperature with a thermocouple 10. The flow-in and flow-out of the molten salt of the ion- exchange bath 1 are controlled to be balanced accurately by the action of the valves 5, 7 which are operated by the controller 16 according to the signal transmitted from the liquid level detector 12.