To provide an illumination optical system that is uniform in brightness on an image display element, is not substantially affected by unevenness of light emission distribution of light source means, and has high use efficiency of light while minimizing angle distribution in a cross section at which angle distribution of an arbitrary optical element is sensitive.

An illumination optical system includes dividing means that divides a light beam from light source means into a plurality of light beams and superimposing means that superimposes the divided light beams on an image display element for illumination. The dividing means includes first light beam dividing means that forms a plurality of first light source images in a first cross section parallel to an optical axis, second light beam dividing means for forming a plurality of second light source images in a second cross section vertical to the first cross section, a compression optical system that narrows a width of the entering light beams for emission, and an optical element having a polarization separation face, a width of a region where the second light source images are formed in the second cross section is smaller than a width of a region where the first light source images are formed in the first cross section, and the compression optical system includes an optical element having positive refractive power and an optical element having negative refractive power.