It is possible to reduce the slightest illumination irregularities generated after assembling an optical system. For this, an illumination optical system includes a light source (11) emitting extreme ultra-violet light, a collimator (12), a Fly’s eye mirror (13), and a capacitor (14) which are successively arranged. A predetermined illumination area in the emission side of the capacitor (14) is subjected to Köhler illumination. At least one of the unit mirrors constituting the Fly’s eye mirror (13) is a correction mirror having reflectivity irregularities for correcting a part or all of the illumination irregularities in the aforementioned illumination area.
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