To provide a lighting optical system and an exposure device capable of reducing the effec of background light.

The lighting optical system includes a diffraction optical element 4 which converts the light intensity distribution of a luminous flux from an optical source 1, multi-luminous flux forming part 12 for uniformly lighting an original 15 using a luminous flux which has passed through the diffraction optical element 4, and a light shielding member 8 arranged on a fourier transform surface 7 which is optically in a fourier transform relation with the diffraction optical element 4 between the multi-luminous flux forming part 12 and the diffraction optical element 4 or in the vicinity of the fourier transform surface. The light shielding member 8 has an opening 8a and a light shield 8b. A border 8c of the opening 8a and the light shield 8b is set to a position in which light strength is 0 at the rising of light strength distribution formed on the fourier transform surface 7 by +1 diffraction light and -1 diffraction light of the diffraction optical element.