An optical integrator used in an illumination optical system for illuminating an illumination target surface on the basis of light from a light source has a first fly's eye optical system having a plurality of first optical elements arranged in parallel at a position optically conjugate with the illumination target surface in an optical path between the light source and the illumination target surface, and a second fly's eye optical system having a plurality of second optical elements arranged in parallel so as to correspond to the plurality of first optical elements in an optical path between the first fly's eye optical system and the illumination target surface. At least one first optical element out of the plurality of first optical elements, and another first optical element different from the at least one first optical element have their respective postures different from each other about an optical axis of the illumination optical system or about an axis parallel to the optical axis.