PURPOSE: To easily treat a porous body as a precursor of glass or ceramics without cracking, to precisely control the distribution and shape of a metallic component in the porous body and to easily obtain a glass body having a refractive index distribution and ensuring high reproducibility and reliability in optical characteristics such as refractive index, difference in refractive index and dispersion characteristics.

CONSTITUTION: When a porous body is dipped in a soln., dried and fired to produce glass or ceramics, the soln. is prepd. by uniformly suspending at least two phases. The 1st phase is a liq. phase and the 2nd phase is one selected from vapor, liq. and solid phase.