An optical device includes: a substrate; an optical waveguide that forms a Mach-Zehnder interferometer; a signal electrode; and a ground electrode. The optical waveguide is placed between the signal electrode and the ground electrode. An electric field is generated in a direction along a surface of the substrate when a voltage is applied between the signal electrode and the ground electrode. The optical waveguide includes a first waveguide through which input light propagates, a curved waveguide which is optically coupled to the first waveguide, and a second waveguide which is optically coupled to the curved waveguide. The signal electrode includes first and second electrodes that are respectively placed near the first and second waveguides. An electric signal is supplied to the first electrode, and an inverted electric signal is supplied to the second electrode.