Halides of Si and Ti, B, P, or Ge and oxygen or steam are introduced into a reaction vessel and heated in a vapor phase to form fine glass particles by oxidation or hydrolysis. The fine glass particles are deposited on a substrate. The deposited fine glass particles are heated and vitrified into a transparent glass layer, which is etched to form a core having a desired pattern by a reactive sputter etching process using Freon gas. The core is coated by a clad. In a waveguide thus formed, the cross sectional configuration and dimensions of the core layer and the refractive index difference are precisely controlled. The waveguide is manufactured with good reproducibility. The fabrication method is suitable for mass production of waveguides. An expansion coefficient transient layer is provided between the core layer and the substrate to prevent a crack in the waveguide. The glass softening temperature of the cladding layer is set lower than that of the core layer, so that the cross sectional configuration and dimensions of the core layer is unchanged against the heat in the vitrification process of the cladding layer. The refractive index is reduced in the vicinity of the boundary between the core and clad to prevent the loss of light due to light scattering on the core side faces.