An illumination optical system includes a light source for emitting a plurality of different colors of light, a plurality of rotating optical scanners, and a plurality of scanning lenses. Each of the optical scanners has a reflecting surface that is formed helically around a cylindrical body. The different colors of light enter the respective reflecting surfaces in the direction parallel to the rotation axis, are reflected therefrom, and pass through the scanning lenses, so that each color of light is magnified and then scans the region to be illuminated sequentially. This makes it possible to provide an illumination optical system that can improve scanning linearity, lower noise because of its reduced wind resistance, and achieve reduction in the cost and power consumption.