A plurality of first electrodes are disposed on one of opposite surfaces of a nonlinear optical material substrate which is divided into single domains. Each of the first electrodes (11) is made of an electric conductor having a width which is equal to or smaller than half the width of a designed local inverted-polarization domain. A second electrode (12) is positioned on the other of the opposite surfaces in confronting relation to the first electrodes. A voltage is applied between the first and second electrodes thereby to produce local inverted-polarization domains in the nonlinear optical material substrate.