To provide an illumination optical system for providing illuminating luminous flux having isotropic NA distribution with compact constitution equipped with a laser array light source.

The illumination optical system IL1 for illuminating the image display surface 10a of a display element 10 has a laser array light source 1 which emits illuminating light having a flat beam cross section, and a rod integrator 8 whose cross-sectional shape is quadrate and which uniformizes spatial energy distribution of the illuminating light. When the illuminating light is made incident on the rod integrator 8, the longitudinal direction of the beam cross section of the illuminating light and the side direction of the cross-sectional shape of the rod integrator 8 form a predetermined angle which is neither parallel nor perpendicular on a cross section perpendicular to the incident direction.

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