An optical modulator is provided with an optical waveguide including an input-side single-mode waveguide for propagating inputted light at single mode, a multi-mode waveguide for propagating the light propagated from the input-side single-mode waveguide at plural guided modes, and an output-side single-mode waveguide for propagating the light propagated from the multi-mode waveguide at single mode to output the light. A lower electrode layer is disposed below the multi-mode waveguide, and an upper electrode is disposed above the multi-mode waveguide. The upper electrode cooperates with the lower electrode layer to apply an electric field to the multi-mode waveguide. A phase of each of the guided modes propagating through the multi-mode waveguide is modulated by applying an electric field to the multi-mode waveguide, and intensity of the light outputted from the output-side single-mode waveguide is modulated by an interference of the phase-modulated guided modes.