A first branched optical waveguide and a second branched optical waveguide, to constitute a Mach-Zehnder type optical waveguide, are formed at the surface of a substrate. A first ground electrode, a signal electrode and a second ground electrode are provided on a buffer layer formed on the substrate. The second ground electrode is partially cut away and divided, to form a ditch therein, so that the modulating electrode composed of the signal electrode, the first and the second ground electrodes are substantially symmetrized on the center line between the first and the second optical waveguides. Then, the ratio (d2/d1) of the distance d2 between the signal electrode and the second branched optical waveguide to the distance d1 between the signal electrode and the first electrode is set within 3.5-7.5.