To provide a lighting optical device which can prevent a lighting pupil or an optical element arranged at its conjugated position from being damaged even if the light energy of an illuminating light emitted from a light source becomes high.

The lighting optical device is provided with a surface light source size variable means 90 that is arranged in an optical path between a light source 1 and an optical integrator 8, and changes the size of a surface light source to be formed at a lighting pupil position by the optical integrator; and an aperture diaphragm 12 that is freely arranged on the incident surface or the outgoing surface of the integrator, or in their vicinity, and has an aperture to limit the surface light source at a predetermined size. It is also provided with a switching means 30 that stops changing operation for the surface light source size by the surface light source size variable means when the size of the surface light source to be changed by the surface light source size variable means becomes smaller than the predetermined size, and switches to surface light source size setting operation using the aperture diaphragm.