PURPOSE: To efficiently form a ridge with excellent forming accuracy by forming a high-refractive index layer on a substrate, cutting two pieces of grooves having a prescribed interval with each other on the surface of the layer by a cutting means, thereby forming the ridge.

CONSTITUTION: A thin film of LnNbO3 having a high refractive index is formed on a single crystal substrate of LiTaO3 to form a material 1 for an optical waveguide. On the other hand, a grinding wheel 2 for cutting fixed with diamond particulates on its circumferential part is prepd. The grinding wheel 2 for cutting is rotated at a high speed while cooling water contg. several % of ≤0.5μm abrasive grains is used to cut the desired part on the surface of the material 1 by as much as a required depth as a ridge height. The material 1 is moved at a low speed by the required length, by which the groove 3 is worked. The grooving is executed twice by changing the position, by which the ridge 4 having 3μm width and 100μm height is exactly formed. The rotation of the cutting means and the feed and position of the work, etc., are precisely controlled in submicron unit, by which the ridge is precisely worked and light propagation loss is lessened.