PURPOSE:To enable formation of an electronic element on a substrate on which an optical circuit element has been formed without being restricted in relation to the process by retarding diffusion of dopant to the silicon substrate side by providing a diffusion inhibiting layer. CONSTITUTION:An inhibiting layer 2 for diffusion of dopant comprising SiO2 film is formed by oxidizing thermally the surface of a silicon wafer 1. The silicon wafer is a P type CZ wafer having (100) face bearings and 9OMEGA-cm resistivity. The thermal oxidation is carried out in dry CO2 atmosphere at 1,000 deg.C and the thickness of the SiO2 film is regulated to ca 1,000Angstrom . Succeedingly, gaseous starting material for forming glass consisting primarily of SiCl4 contg. appropriate amt. of GeCl4, BCl3, PCl3 as dopant is converted to fine glass particles by the frame hydrolysis of the gaseous starting material to deposit film 3 of fine glass particles having ca. 700mum thickness, which is heated in an electric furnace (in oxidizing atmosphere) to 1,150 deg.C and held for 2hr to vitrify the fine glass particle film 3 to transparent glass. Thus, optical waveguide film 4 is obtd. on the silicon wafer 1.