An illumination optical system is disclosed, which leads a luminous flux forward effectively, which emitted backward from a light source. The illumination optical system comprises a light source, and a first optical member arranged on an opposite side of a light irradiation side with respect to the light source. The first optical member includes a refractive surface through which light from the light source is transmitted and a reflective surface which reflects the light from the refractive surface toward the light irradiation side.