Abstract: A method for fabricating an optical wavelength converter element which has a high optical damage threshold value, and is capable of achieving a high wavelength conversion efficiency without involving propagation losses caused by the reflection and scattering of a fundamental wave and a wavelength-converted wave. Domain reversals periodically arranged are formed on a substrate made of a LiNbO.sub.3 crystal which acts as a ferroelectric substance possessing a nonlinear optical effect. Thereafter, an ITO thin film made of a transparent electrically conductive material is deposited on a surface of the substrate extending in the direction of the arrangement of the domain reversals, that is, "y" and "z" surfaces.