To provide an illumination optical system capable of illuminating a surface to be illuminated with light of a required polarization state by suppressing influence of retardation caused by a subsequent optical system of a polarization conversion member.

The illumination optical system illuminates a surface to be illuminated with light that passes an illumination pupil by distributing light from a light source across the illumination pupil. The illumination optical system includes a polarization conversion member, disposed in an optical path of the illumination optical system, and converting a polarization state of incident light to change light distributed across the illumination pupil of the illumination optical system into a predetermined polarization state, a subsequent optical system, disposed between the polarization conversion member and the surface to be illuminated, and changing linearly polarized light polarized in a first direction among light distributed across the illumination pupil into elliptically polarized light, and a phase modulation member, disposed in the optical path of the illumination optical system, maintaining a polarization state of linearly polarized light polarized in a second direction obliquely crossing the first direction, and converting the linearly polarized light polarized in the first direction into an elliptically polarized light so as to reduce ellipticity of polarized light changed into elliptical polarization.