Phenomenon of silica-fiber loss increase, which is induced by an electric current applied in water, is reported. It is found from experimental results that the loss increase is caused by hydrogen molecules, which are electrolytically produced and are diffused into the fiber interstitially. Furthermore, both infrared loss and Ramanscattering spectra are measured for a fiber dissolved by H2, HD, and D2 molecules to make clear assignment for loss peak. As a result, it is found that the absorption loss peaks are associated with a connection between molecular vibration and SiO4 tetrahedral vibrations in the silica glass.