PURPOSE: To form a glass film for an optical waveguide having a homogeneous high-refractive-index glass thin film layer on the surface of a glass waveguide.

CONSTITUTION: A gas contg. the org. oxysilane 15 contg. a refractive index controlling dopant 14 and oxygen is blown against a substrate 6 with the surface covered with a high-refractive-index glass film in a low-temp. plasma atmosphere in vacuum to form a high-refractive-index glass thin film contg. the dopant 14 on the glass film surface of the substrate 6. In this case, an inert gas is added from the surround of the plasma atmosphere to form a glass film for an optical waveguide to stabilize the plasma. Further, the substrate coated with the low-refractive-index glass film is preferably kept at 100-400°C in the plasma atmosphere.