An optical system illuminating a surface to be illuminated includes a wavefront splitting type integrator configured to split the wavefront of incident light to form a plurality of light sources on the exit surface side, and an optical element whose surface is polished in a scanning direction using a polishing tool. The optical element is disposed between the wavefront splitting type integrator and the surface to be illuminated, and has a direction indicating portion indicating the scanning direction. The arrangement direction of the plurality of light sources in a plane perpendicular to the optical axis of the optical system is non-parallel to the scanning direction indicated by the direction indicating portion.