To build an optical core in such a manner that an optimum objective region encloses a core layer and prevents the outer diffusion of a doping material by fluorinating the just deposited layer before depositing the cover layer of a core structure.

A lower layer 2 thicker than the core is first produced on a substrate 1 surface. This lower layer 2 is formed of silicon oxide doped with, for example, boron oxide and is sintered on a wafer of, for example, silicon. The surface of the lower layer 2 is etched with a soln. contg. the fluoride. Next, the core layer 3 of the silicon oxide is deposited on a boundary layer 4 contg. the fluoride. This glass layer 3 is doped with, for example, the boron oxide in order to increase the refractive index of glass material. Net, the entire part of the structure surface including the lower layer 2 and a waveguide structure 3 is treated with the soln. contg. the fluoride. As a result, the silicate layer is slightly removed and the boundary layer is fluorinated. The upper cover layer 5 is deposited after the formation of the fluoride layer.