To reduce influence of irregularities of emission intensity distribution of each emission part and emission intensity between a plurality of the emission parts to uniformly illuminate illuminated faces by having light sources including the plurality of the emission parts.

An illumination optical system 1 includes: the light sources 11 including the plurality of the emission parts 11a, 11b, 11c arranged in a specific direction; and an integrator 21 including a plurality of lens cells arranged in the specific direction. The illumination optical system emits light from the plurality of the emission parts to illuminate the illuminated face 31 with light fluxes through the integrator. A plurality of the light fluxes from the plurality of the emission parts mutually have shift in the specific direction as a dispersion light fluxes each widened in the specific direction to be incident on an incident face of the integrator. The two light fluxes of the two mutually adjacent emission parts among the plurality of the emission parts mutually overlap in a region Lm larger than one lens cell on the incident face of the integrator.