An optical device having a Lambertian light distribution closer to a linear or quadrangular light distribution is provided. An optical device is provided that comprises: a first reflecting surface disposed so as to reflect first light that has a light distribution having an optical axis parallel to a first axis, to an arc-shaped first region surrounding the first axis; and a second reflecting surface and a third reflecting surface that are disposed such that the second reflecting surface and the third reflecting surface meet each other on the first axis, and such that the first reflecting surface is disposed between the second reflecting surface and the third reflecting surface.