An illumination optical system for illuminating an illumination target surface (M; W) with illumination light from a light source, said illumination optical system comprising a spatial light modulator having a plurality of mirror elements which are arrayed two-dimensionally and conditions of which are controlled individually, characterized by: a distribution forming optical system arranged to form a predetermined light intensity distribution on an illumination pupil of the illumination optical system, based on the illumination light having traveled via at least a part of the plurality of mirror elements of the spatial light modulator; and an inspection device arranged to inspect the spatial light modulator, the inspection device comprising: a splitting optical member which is arranged in an optical path of the illumination optical system to split part of the illumination light having traveled via at least a part of the plurality of mirror elements of the spatial light modulator; a Fourier transform optical system arranged to form a Fourier transform plane optically in a Fourier transform relation with an array plane where the plurality of mirror elements are arrayed by light from the splitting optical member; a photodetector having a detection surface arranged on or near the Fourier transform plane; and an inspection unit arranged to inspect optical characteristics of at least one mirror element of the plurality of mirror elements using a first light intensity distribution and a second light intensity distribution, the first light intensity distribution being formed on the detection surface by light from the plurality of mirror elements when the plurality of mirror elements is set in a first state, the second light intensity distribution being formed on the detection surface by light from the plurality of mirror elements in a second state in which a posture of said at least one mirror element is changed. a distribution forming optical system arranged to form a predetermined light intensity distribution on an illumination pupil of the illumination optical system, based on the illumination light having traveled via at least a part of the plurality of mirror elements of the spatial light modulator; and an inspection device arranged to inspect the spatial light modulator, the inspection device comprising: a splitting optical member which is arranged in an optical path of the illumination optical system to split part of the illumination light having traveled via at least a part of the plurality of mirror elements of the spatial light modulator; a Fourier transform optical system arranged to form a Fourier transform plane optically in a Fourier transform relation with an array plane where the plurality of mirror elements are arrayed by light from the splitting optical member; a photodetector having a detection surface arranged on or near the Fourier transform plane; and an inspection unit arranged to inspect optical characteristics of at least one mirror element of the plurality of mirror elements using a first light intensity distribution and a second light intensity distribution, the first light intensity distribution being formed on the detection surface by light from the plurality of mirror elements when the plurality of mirror elements is set in a first state, the second light intensity distribution being formed on the detection surface by light from the plurality of mirror elements in a second state in which a posture of said at least one mirror element is changed. a splitting optical member which is arranged in an optical path of the illumination optical system to split part of the illumination light having traveled via at least a part of the plurality of mirror elements of the spatial light modulator; a Fourier transform optical system arranged to form a Fourier transform plane optically in a Fourier transform relation with an array plane where the plurality of mirror elements are arrayed by light from the splitting optical member; a photodetector having a detection surface arranged on or near the Fourier transform plane; and an inspection unit arranged to inspect optical characteristics of at least one mirror element of the plurality of mirror elements using a first light intensity distribution and a second light intensity distribution, the first light intensity distribution being formed on the detection surface by light from the plurality of mirror elements when the plurality of mirror elements is set in a first state, the second light intensity distribution being formed on the detection surface by light from the plurality of mirror elements in a second state in which a posture of said at least one mirror element is changed.