An optical modulator including a substrate made of lithium niobate having an electrooptical effect, first and second optical waveguides formed in the substrate by effecting a thermal diffusion of titanium into a surface of the substrate, a buffer layer formed on the surface of the substrate, a hot electrode provided on the buffer layer and having a width W.sub.e not larger than a width W.sub.f of the optical waveguide, first and second ground electrodes provided on the buffer layer such that these ground electrodes are symmetrical with respect to the hot electrode, and an electric field adjusting region having a width not smaller than that of the hot electrode is provided between the buffer layer and the hot electrode. First and second ground side electric field adjusting main- and sub-regions may be provided between the buffer layer and the first and second ground electrodes, respectively. An interaction between an electric field generated by applied microwave and lightwave propagating along the optical waveguides can be enhanced, and thus a driving voltage can be decreased.