An optical device includes a plurality of optical waveguides, and a planar optical waveguide. The plurality of optical waveguides each extend in a first direction, and are arranged in a second direction intersecting the first direction. The planar optical waveguide is connected directly or indirectly with the plurality of optical waveguides. The plurality of optical waveguides each allow light to propagate in the first direction. The planar optical waveguide includes a first mirror and a second mirror, and an optical waveguide layer. The first mirror and the second mirror face each other, and extend in the first direction and the second direction. The optical waveguide layer is located between the first mirror and the second mirror.