A method for manufacturing deuterium-treated silica glass includes exposing silica glass to a deuterium-containing atmosphere for a predetermined period of time to diffuse deuterium molecules within the silica glass, maintaining the silica glass at 40° C. or higher, and cooling the silica glass to room temperature. The silica glass is a silica glass-based optical fiber having a core made of silica glass, where the core is positioned at a center of the optical fiber and contains at least germanium, and a clad made of silica glass, where the clad surrounds the core and has a lower refractive index than the core. A surface of the silica glass is covered with a resin coating.