PURPOSE: To prepare a photo focusing lens at high temperature without causing the deformation, by keeping a glass rod with its surroundings covered with a porous cushioning material in the interior of a substrate, followed by immersing the rod in a molten salt bath while rotating it together with the substrate.

CONSTITUTION: The top and the bottom discs 11 and 12 are connected by the four bars 13', the glass rod 1 is put in the substrate 2 equipped with the rotating axes 13 and 13 at its top and bottom, and the porous cushioning material (e.g., stainless steel fibers) 3 is packed around the glass rod 1. The cushioning material 3 is surrounded by the stainless steel wire cloth 4, and the glass rod 1 is supported approximately in parallel with the rotating axes 13 and 14. The above-mentioned substrate 2 is immersed in the molten salt 7 in the bath 5, and the glass rod 1 is subjected to ion exchange treatment by the molten salt 7 through the cushioning material 3, while being rotated at about 10W2,000rpm in the direction of the arrows around the rotating axes 13 and 14.