The intensity change in optical absorption at 3.8, 5.0, and 5.8 eV was measured for thermal treatment up to 1000 °C done on silica glass samples manufactured by various methods. It was found that these absorption bands change differently with heat, depending on the synthesis process of the sample. The 3.8‐eV absorption band due to peroxy linkage is found to increase with high‐temperature treatment when the sample contains dissolved oxygen molecules within the sample. For the sample with no oxygen molecules, high‐temperaturetreatment does not alter the absorption intensity at 3.8 eV. The 5.0‐eV absorption band is found to decrease by high‐temperature treatment. In the case of an oxygen‐containingsample, the reaction of oxygen molecules with oxygen vacancy sites is a cause of this decrease. The 5.1‐eV band, which was thought to have no temperature dependence, is found to be annealed at a temperature region around 950 °C.