A lower clad, cores, and an upper clad are formed by a chemical vapor deposition
method (CVD method). At least one of the additional amount of oxygen, the additional
amount of nitrogen, and the additional amount of silicon of a silicon oxynitride
film is adjusted so that the cores have a desired higher refractive index than
those of the clads. Further, end point detectors are formed which become etching
stoppers of dry etching in pattern forming the cores.