An optical device includes a first waveguide layer in which multiple-beam interference occurs, a second waveguide layer which includes a back surface facing the first waveguide layer and a front surface as a light output surface, and a diffraction grating which is arranged on a back side of the second waveguide layer and faces the first waveguide layer, wherein a grating constant of the diffraction grating is defined such that a first-order diffracted light emerges from the second waveguide layer, the first-order diffracted light being generated when a light component having a highest intensity of light which propagates in an in-plane direction while causing multiple reflection in the first waveguide layer enters the diffraction grating.