An optical device includes a modulator and a tap coupler. The modulator includes an optical waveguide that is formed of a thin-film lithium niobate (LN) substrate and through which light passes, and an electrode that applies voltage to the optical waveguide, and modulates a phase of light that passes through the optical waveguide in accordance with an electric field in the optical waveguide, where the electric field corresponds to the voltage. The tap coupler includes at least a part formed of the thin-film LN substrate, and splits a part of the light that passes through an inside of the optical waveguide. The tap coupler includes a delayed interferometer that splits a part of the light that passes through the optical waveguide, at a split ratio corresponding to a phase difference of light that passes through an inside of the tap coupler from the optical waveguide.