To obtain an illumination optical system that has brightness even on the image display elements and is highly efficient in using light without being substantially affected by the uneven distribution of the light emitted from the light source although reducing the angle distribution of the optical elements on its cross section sensitive to the angle distribution.

This illumination optical system has a divider to divide the light flux from the light source into several light fluxes, and a superposition means to superpose the divided light fluxes on the image display elements for illuminating. The divider has a first light flux divider to divide the light flux from the light source in the first cross section parallel to the optical axis and to form several first light source images in the first light source area by using the divided light fluxes, and a second light flux divider to divide the light flux from the light source in the second cross section parallel to optical axis and perpendicular to the first cross section and to form several second light source images in the second light source area by using the divided light fluxes. The compression optical system disposed in the light source side in the second light flux divider is built appropriately.