A reduction in size and cost of an optical modulator is achieved with a simple configuration, while improving the modulation characteristics. An optical modulator modulates light branched by an optical coupler and then couples the light via the optical coupler. The optical coupler is formed in a substrate having electro-optic effects. An optical waveguide is formed in the substrate and, includes a turnback section and ends into which the light branched by the optical coupler is input. A signal electrode is provided in the substrate along the optical waveguide. A modulation signal to modulate the light passing through the optical waveguide is input to the signal electrode.