The present invention provides a glass optical waveguide device that comprises silica glass and is usable despite a relatively large refractive index difference between its core layer and cladding layer, and a method of manufacturing such device. This optical waveguide device includes a core layer on or over a substrate and a cladding layer that covers the core layer, wherein the core layer is made of silica glass containing an additive for raising the refractive index, and the relative refractive index difference of the core layer with respect to the cladding layer is at least 2.5%. The method of manufacturing this optical waveguide device includes the steps of placing a substrate over an electrode plate that is connected to a first high-frequency power supply within a vacuum vessel, introducing a raw material gas into the vacuum vessel and generating a plasma within the vacuum vessel with high-frequency power supplied from a second high-frequency power supply, and simultaneously forming an optical waveguide film over the substrate while supplying high-frequency power from the first high-frequency power supply to the electrode plate.