An illumination optical system for illuminating an object surface using light from a light source, the illumination optical system includes, a first optical system that includes a movable unit that is movable along an optical axis, said first optical system guiding the light to the object surface and varying an irradiation area on a certain plane, and a second optical system that can be located on and removed from an optical path of said first optical system, the second optical system varies, when located on the optical path of said first optical system, the irradiation area on the certain plane in cooperation with said first optical system, while maintaining a shape of a light intensity distribution on the certain plane, relative to the irradiation area irradiated only by said first optical system.