There are provided an illumination optical apparatus and an optical apparatus using this illumination optical apparatus that are capable of efficiently collecting light emitted from a light-emitting section for illumination with less illumination nonuniformity, without having to employ a complicated structure. There are provided a light-emitting sections; a lens system for converting a diverging beam emitted from the light-emitting section into a beam of collimated light; an afocal optical system for adjusting the cross-sectional area of a beam of collimated light obtained by the lens array; a fly-eye lens for forming a plurality of light-source images from the collimated light whose cross-sectional area is adjusted by the afocal optical system; and a Koehler illumination optical system that uses the plurality of light-source images formed by the fly-eye lens as a light source.