An illumination system includes a light source, first and second cylindrical lens arrays, and a relay lens. The first cylindrical lens array includes a plurality of cylindrical lens cells which divide light emitted from the light source into a plurality of beams. The second cylindrical lens array includes a plurality of cylindrical lens cells which combine the beams divided by the cylindrical lens cells in an identical direction. The relay lens relays beams passed through the second cylindrical lens array so that most of the beams are concentrated on an incident light axis to have a Gaussian distribution. Accordingly, light passed through slits for controlling the divergence angle of incident light or the etendue of an optical system has a Gaussian distribution in a color separation direction or a color scrolling direction.