Silica soots were sintered under a reducing ambient to study the generation mechanism of the oxygen‐deficient‐type defects in silica glasses. Glasses sintered under H2/He gas mixtures or vacuum were prepared with an intent to produce oxygen‐deficient centers. Two optical‐absorption bands were generated in the silica glasses sintered under the H2/He gas mixtures. One was 5.17‐eV absorption band and the other was an absorption tail near the Urbach tail of silica glasses. Both absorptions were attributed to the silicon lone pair center (SLPC). On the other hand, the other type of reduced silica glasses were prepared in the vacuum ambient. Three kinds of absorption bands could be detected clearly: one was peaking at 5.17 eV, another showed only its shoulder near the Urbach tail, (both were also attributed to the SLPC) and the third was peaking at 6.7 eV and was attributed to a SiSiSi structure. Fabricated in higher volatile ambient under vacuum, the color of the sample showed a smoky black and generated broad absorption band peaking at 5.1 eV. This band is tentatively attributed to the optical absorption due to excitation from the inner shell of silicon.