An illumination optical system comprises plural rod integrators, a combining optical system and a light transmission unit. The rod integrators uniformize light intensity distributions of the light beams. The combining optical system combines the light beams emitted from the rod integrators, so that the light beams are adjacent to each other in cross-sections thereof. The light transmission unit has an entrance plane and an exit plane, divides the light beam combined by the combining optical system into plural light beams on the entrance plane, joins the divided light beams so that a cross-sectional shape of a joined pattern of the light beams on the exit plane is different from a cross-sectional shape of a joined pattern of the light beams on the entrance plane, and transfers each of the light beams from the entrance plane to the exit plane using plural optically coupled light pipes.