A Mach-Zehnder optical waveguide is disposed in a substrate made of a material having an electro-optic effect. Coplanar-waveguide modulating electrodes are disposed on a principal surface of the substrate. A dielectric layer is disposed on a reverse surface of the substrate. A supporting substrate having a recess is disposed in contact with the dielectric layer such that the recess is located at a position corresponding to a modulating region. The relationship: _rs is satisfied where _r represents the dielectric constant of the supporting substrate and _s represents the dielectric constant of the dielectric constant of solid, liquid, or gaseous substance in the recess.