PURPOSE: To make it possible to obtain a wide band width in both of a low switching voltage and good impedance matching by using an X-cut substrate of lithium niobate and allowing the thickness of electrodes and the thickness of a buffer layer to exhibit performance cooperatively with a gap width, inter- optical channel spacings and thermal electrode width.

CONSTITUTION: The two grounding electrodes 120, 124 and the one thermal electrodes 122 are arranged on a lithium niobate substrate. These electrodes are separated from the lithium niobate substrate by the buffer layers 222. The buffer layers 22 are silicon dioxide. Air exists above and around the electrodes in the gaps 126. The optical waveguide channels 230 exist below the gaps 126 and lower than the surface of the substrate. The width Wg of the grounding electrodes are formed in order of the thermal electrode width Wh. The thickness D of the buffer layer is increased and the thickness of the electrodes is increased. The gap width G is formed wider than the gap width of the optical modulator and the thermal electrode width Wh is formed narrow.