To homogenize the partial, dispersion of refraction factor caused by a SiO2 fine particle incorporated at the time of synthesis and different in the composition and the form by thermally treating an optically heterogeneous quartz glass at a respectively specified treating temp., gas atmosphere, pressure and temp. descending rate.

The optically heterogeneous quartz glass is thermally treated at 1200-2200°C treating temp., in an inert gas, a combustion aid gas, gaseous hydrogen or the gaseous mixture atmosphere under 0-10 kg/cm2 pressure or a vacuum at 5-100°C temp. descending rate. As a result, the quarts glass 1×10-8 to 6×10-7 in the difference (data) Δn of the refraction factor between the partial heterogeneity and the surroundings and ≤2 nm/cm in stress is obtained. That is, the refraction factor of the fine particle part at the normal temp. is consequently held dot to be the same as that of the surroundings by gradually lowering the glass temp. to the normal. temp. before the SiO2 fine particle is fixed to have the original refraction factor.