To provide an illumination optical system achieving high-grade resolution with a simple configuration, and to provide an exposure apparatus.

In the illumination optical system that is used for the exposure apparatus for exposing a pattern on a precursor to a substrate using light from a light source, irradiates the precursor with light, and has a section for adjusting the amount of exposure, the section for adjusting the amount of exposure has a pair of optical element groups capable of varying the interval among the groups while each group includes one optical element or more, and an aperture diaphragm arranged at the side of the precursor as compared with the pair of optical element groups in the optical path of the illumination optical system. In the illumination optical system, the luminous flux diameter of light through the pair of optical element groups is changed since the interval of the pair of optical element groups changes, and spread in the aperture diaphragm of rays emitted from a point at a finite or infinite distance is changed.