An illumination optical system which is used with a reflective imaging optical system configured to form an image of a pattern arranged on a first plane onto a second plane, and which illuminates an illumination area on the first plane with a light from a light source. The illumination optical system includes one or more reflecting mirrors configured to reflect the light from the light source such that the light from the light source passes between first and second mirrors of a plurality of mirrors provided in the reflective imaging optical system, the first mirror being configured to reflect the light from the pattern first, and the second mirror being configured to reflect the light from the pattern second.