PURPOSE: To lessen the temp. rise of a crystal plate and to lessen light loss, phase deviation and waveform distortion by connecting coupling electrodes to signal electrodes provide on an optical waveguide and connecting an introducing electrode to the coupling electrodes.

CONSTITUTION: The main surface of a Z-cut crystal plate 1 consisting of lithium niobate is provided thereon with the optical waveguides 2 of Mach-Zehunder type. The one optical waveguide 2 is provided therein with a grounding electrode 3 consisting of gold (Au) of 3μm thickness and the other optical waveguide 2 is provided thereon with the signal electrode 4 consisting of gold. The optical waveguides 2 are formed by depositing titanium by evaporation on the parts corresponding to the optical waveguides 2 on the main surface of the crystalline plate 1 by a photolithography method, then subjecting these parts to a thermal diffusion treatment at 1050°C in an oxygen atmosphere. Five pieces of the coupling electrodes 11 consisting of gold and having 20μm length × 10μm width × 3μm thickness are provided in connection to this signal electrode 4 and further the introducing electrode 12 consisting of gold and having 20μm length × 27μm width × 3μm thickness are provided in connection to the coupling electrodes 11. Then, the internal impedance of the signal electrode is lowered without changing the impedance between the signal electrode and the grounding electrode.