An illumination device comprises: an optical member that is provided with a circular ring-shaped or horseshoe-shaped light-guiding layer and diffusion layer, which are laminated in a central axis direction, the light-guiding layer having a light-entrance surface facing a tangential direction； and a light-introducing member that is disposed at the radially outer side of the optical member and that introduces illumination light into the light-guiding layer from the light-entrance surface, in the tangential direction； wherein the diffusion layer diffuses the illumination light entering from the light-guiding layer by volume scattering, and the optical member is formed, at least, of one end surface in the axial direction and emits the illumination light emitted from the diffusion layer.
