A reflection type illumination optical apparatus, which guides an exposure light to a reticle surface via a curved mirror, a concave mirror, etc. includes a vacuum chamber which accommodates the curved mirror, the concave mirror, etc; and a subchamber which is arranged in the vacuum chamber and which accommodates the curved mirror. The subchamber has openings through which the exposure light coming into the curved mirror and the exposure light exiting from the curved mirror pass, respectively. Each of the openings is arranged in the vicinity of a position at which the cross-sectional area of the light flux is smallest. It is possible to decrease the amount of adhesion of minute particles such as debris to the reflecting optical element, without unnecessarily enhancing the ability of the vacuum gas discharge mechanism.