To provide an illumination optical device capable of forming annular illumination pupil distribution in a circumferential polarization state while suppressing light quantity loss to an excellent level.

The illumination optical device comprises a light flux conversion element (50) to form an annular light intensity distribution on a predetermined plane based on an incident luminous flux. The light flux conversion element formed from an optical material having optical rotation comprises: a first basic element (50 A) to form a first arcuate area distribution of the annular light intensity distribution based on the incident luminous flux; a second basic element (50B) to form a second arcuate area distribution; a third basic element (50 C) to form a third arcuate area distribution; and a fourth basic element (50 D) to form a fourth arcuate area distribution. Each basic element has thickness different to each other along a light transmission direction.