Abstract: Using an LiTaO.sub.3 or LiNbO.sub.3 substrate, a proton exchange layer in a grid pattern, namely sprout areas of polarization inversion, is firstly, formed on the surface of said substrate and, after formation of the pattern, heat treatment is executed at a temperature of 200.degree. C. or more and for a holding time of 10 minutes or less. By maintaining the temperature gradient up to said heat treatment point at 50.degree. C./min. or steeper and by maintaining the temperature decrease rate from said heat treatment point at 50.degree. C./min. or faster, polarization inverted areas are formed downwards from the proton exchanged areas, while in addition to making the top ends of said polarization inverted areas into an acute angle, the depth/weight ratio of the polarization inverted grids being formed is made to exceed 1, thus enabling the production of a high-efficiency SHG element.