An illumination optical apparatus for illuminating a pattern on a mask with illumination light, the illumination optical apparatus comprising: an optical integrator arranged in an optical path of the illumination light, the optical integrator being a fly's eye lens or micro fly's eye lens; and a polarization modulating member which is arranged in the optical path on an incidence side of the optical integrator, and which is arranged for changing a polarization state of the illumination light; wherein: the apparatus further comprises a distribution adjusting optical system (8, 9) which is arranged in the optical path between the polarization modulating member and the optical integrator and which is able to adjust at least a size of the distribution of the illumination light on a pupil plane of the illumination optical apparatus, wherein the polarization modulating member is configured to change the polarization state of the illumination light, being incident on the polarization modulating member in a linearly polarized state, so that a polarization direction of the illumination light in a region on the pupil plane is substantially coincident with an azimuthal direction about an optical axis of the illumination optical apparatus, the region being away from the optical axis, the polarization modulating member being made of an optical material with optical activity and providing different thicknesses of the optical material in a direction of the optical axis at different azimuthal positions about the optical axis, and wherein the polarization modulating member is arranged so that a direction of an optic axis of the optical material is substantially coincident with a direction of the optical axis. an optical integrator arranged in an optical path of the illumination light, the optical integrator being a fly's eye lens or micro fly's eye lens; and a polarization modulating member which is arranged in the optical path on an incidence side of the optical integrator, and which is arranged for changing a polarization state of the illumination light; wherein: the apparatus further comprises a distribution adjusting optical system (8, 9) which is arranged in the optical path between the polarization modulating member and the optical integrator and which is able to adjust at least a size of the distribution of the illumination light on a pupil plane of the illumination optical apparatus, wherein the polarization modulating member is configured to change the polarization state of the illumination light, being incident on the polarization modulating member in a linearly polarized state, so that a polarization direction of the illumination light in a region on the pupil plane is substantially coincident with an azimuthal direction about an optical axis of the illumination optical apparatus, the region being away from the optical axis, the polarization modulating member being made of an optical material with optical activity and providing different thicknesses of the optical material in a direction of the optical axis at different azimuthal positions about the optical axis, and wherein the polarization modulating member is arranged so that a direction of an optic axis of the optical material is substantially coincident with a direction of the optical axis. the apparatus further comprises a distribution adjusting optical system (8, 9) which is arranged in the optical path between the polarization modulating member and the optical integrator and which is able to adjust at least a size of the distribution of the illumination light on a pupil plane of the illumination optical apparatus, wherein the polarization modulating member is configured to change the polarization state of the illumination light, being incident on the polarization modulating member in a linearly polarized state, so that a polarization direction of the illumination light in a region on the pupil plane is substantially coincident with an azimuthal direction about an optical axis of the illumination optical apparatus, the region being away from the optical axis, the polarization modulating member being made of an optical material with optical activity and providing different thicknesses of the optical material in a direction of the optical axis at different azimuthal positions about the optical axis, and wherein the polarization modulating member is arranged so that a direction of an optic axis of the optical material is substantially coincident with a direction of the optical axis.