An object of the invention is to provide a method of producing an optical waveguide comprising a siloxane-containing polymer film, capable of controlling a refractive index easily and finely, and an optoelectronic hybrid substrate using the optical waveguide, capable of controlling a refractive index easily and finely, and reducing the effect of the surface roughness of the substrate. In particular, it relates to a method of producing an optical waveguide comprising a siloxane-containing polymer film containing a metal obtained by the thermal polymerization of a siloxane-containing polymer film formation solution added with a metal alkoxide on a substrate, and an optoelectronic hybrid substrate comprising a combination of an optical waveguide produced as mentioned above, a photoelectric element and an electric circuit on a ceramic substrate.