The illumination optical system is capable of reducing, without moving any optical member and without causing flicker when displaying a still image, sample-and-hold blur when displaying a moving image. The illumination optical system (20) respectively guides multiple light fluxes (Li, Lii, Liii) from multiple light sources (i, ii, iii) in a light source unit (10) to multiple illumination regions (4a, 4b, 4c) on an illumination surface (4). The illumination optical system includes an integrator optical system (1, 2) located between the light source unit and the illumination surface. The integrator optical system includes a first lens array (1) and a second lens array (2) each including multiple lens cells in order from a light source unit side. The illumination optical system changes illumination states of the multiple illumination regions depending on changes of states of the light sources.