An illumination optical system illuminates an irradiated surface with light supplied from a light source. The illumination optical system includes a diffractive optical element disposed in an optical path of linearly polarized light supplied from the light source. The diffractive optical element forms a multipole illumination field including a plurality of illumination fields. The illumination optical system also includes an optical integrator that forms a multipole light source including a plurality of planar light sources with light that has passed through the multipole illumination field. The illumination optical system also includes a polarization optical member disposed in an illumination optical path and that sets a polarization direction of light that has passed through the multipole light source to a predetermined polarization direction.