An illumination optical system which illuminates a plane to be illuminated using light from a light source, includes a first integrator and a second integrator disposed in order from the light source, a diaphragm having an aperture of constant area and disposed between the first integrator and the second integrator and close to the first integrator and an irradiation range adjustment unit configured to adjust an irradiation range of the light from the light source on a plane where the diaphragm is disposed, the irradiation range includes the aperture.