An illumination optical apparatus is able to quickly perform switching between polarization states of illumination light in a first region and in a second region. The illumination optical apparatus of the present invention for illuminating a surface to be illuminated on the basis of light from a light source is provided with an optical integrator of a wavefront division type arranged in an optical path between the light source and the surface to be illuminated and including a plurality of wavefront division regions; and a polarization changing member for changing a polarization state of at least one beam out of a beam incident to a first region in the wavefront division regions of the optical integrator and a beam incident to a second region in the wavefront division regions of the optical integrator.