The present invention provides an illumination device which selectively radiates either one of right-circularly polarized light and left-circularly polarized light, comprising a reflective polarizing plate 1, a light source, and a reflective polarizing plate 2 in this order, and further comprising a retardation plate, wherein the reflective polarizing plate 1, the light source, and the reflective polarizing plate 2 are disposed such that polarized light reflected by the reflective polarizing plate 1 passes through the reflective polarizing plate 2 and polarized light reflected by the reflective polarizing plate 2 passes through the reflective polarizing plate 1, and phase difference and disposition of the retardation plate are adjusted such that the above either one of the circularly polarized lights is radiated toward both of a direction of the reflective polarizing plate 1 and a direction of the reflective polarizing plate 2 based on the light source. The illumination device of the present invention radiates either one of right-circularly polarized light and left-circularly polarized light with good energy efficiency.
