An illumination optical apparatus which illuminates an illumination objective surface with an exposure light includes: an illumination optical system having a curved mirror and a concave mirror and defining a position substantially conjugate with the illumination objective surface between the curved and concave mirrors; and a second aperture plate separating a space in which the curved mirror is arranged and a space in which the concave mirror is arranged into mutually different vacuum environments or pressure-reduced environments, and having an aperture through which the exposure light passes, the aperture being arranged at a position at which a cross-sectional area of the exposure light is smallest, or in the vicinity of the position. It is possible to decrease the amount of passage of minute particles such as debris in relation to any downstream-side optical system.