A technical problem related to a traveling wave electrode type of optical modulator comprising a substrate having the electro-optical effect, optical waveguides formed in the substrate, and a traveling wave electrode formed above the substrate includes improvement of the characteristics such as optical modulation bandwidth, driving voltage, and characteristic impedance of the traveling wave electrode type of optical modulator. To solve the problem, the structure of ridge portions is optimized which is formed in such a manner that a part of the substrate at regions where electric field generated by a high frequency electric signal traveling through the traveling wave electrode is strong is reduced in thickness by digging. Further, a buffer layer is formed over the substrate where the ridge portions are formed and a conducting layer is formed over the buffer layer. The thickness of at least one part of the buffer layer along the normal line of a side surface of the ridge portions is less than the thickness of the buffer layer on a bottom surface between the ridge portions formed by digging and/or the thickness of the buffer layer on a top part of the ridge portions.