In one embodiment, in an optical device, a bar-shaped optical waveguide has either a polygonal or circular cross-sectional shape. A light entry portion is formed in a circumferential area of a first-end surface of the optical waveguide. The light entry portion includes a sloping surface having a normal vector containing a component in a circumferential direction of the first-end surface. An incident light beam travels towards a second-end surface of the optical waveguide while repeating total reflections on a side surface of the optical waveguide. The incident light beam travels without passing through a central portion in a cross section of the optical waveguide. A light exit portion is formed in the side surface of the optical waveguide. The light exit portion is configured to let the light beam in the optical waveguide out of the optical waveguide.