To quantitatively evaluate the amount of each of four- and three- membered-ring structures by obtaining, as a quantitatively index, the ratio of the peak value of the approximation function of peaks D1 and D2 for the peak value of the approximation function of a peak resulting from skeleton vibration.

Approximation is made to determine approximation functions 3-8 by a Lorentz function for peaks D1 and D2 resulting from four- and three- membered-ring structures in the Si-O net structure of scattered light intensity y (Raman spectrum 1) where Raman shift x is within 100-700 cm-1, by a Gaussian function for a skeleton vibration peak, and by a secondary function for background excluding each of them. Calculation is repeated so that a difference 9 between their sum (a Raman spectrum approximation function 2) and the Raman spectrum 1 becomes small, thus calculating the peak value of the approximation function of the skeleton vibration peak. The ratio of the peak values of the approximation functions 4 and 5 of the peaks D1 and D2 for the peak value is obtained as the quantitative index of the amount of four- and three-membered-ring structures included in quartz glass.