An optical device includes a first substrate, a second substrate, a plurality of separation walls, one or more optical waveguides, and one or more spacers. The first substrate has a surface which extends in a first direction and a second direction intersecting the first direction. The second substrate faces the first substrate. The plurality of separation walls are positioned between the first substrate and the second substrate and extend in the first direction. The one or more optical waveguides are positioned between the first substrate and the second substrate and include one or more dielectric members which are positioned between the plurality of separation walls and which extend in the first direction. The one or more spacers are directly or indirectly sandwiched between the first substrate and the second substrate and positioned around the one or more optical waveguides.