In an illumination optical system, a light flux from a light source is made to come into a first fly's eye optical system, and an illumination area is illuminated, via a second fly's eye optical system and a condenser optical system, with light fluxes from a plurality of mirror elements which construct the first fly's eye optical system, wherein a reflecting surface of each of the mirror elements has a width in one direction narrower than a width of each of the mirror elements in a direction perpendicular to the one direction, and a reflectance distribution in the one direction of each of the mirror elements is trapezoidal. The intensity distribution of the illumination area can be set to be a nonuniform distribution, and respective points in the illumination area can be illuminated with the light fluxes having an approximately same aperture angle distribution.