To restrain loss in a quantity of light by making the curvature radii of two lens cells of an eccentric fly-eye lens different from each other.

An illumination optical system comprises: a first lens array having a plurality of lens cells dividing a luminous flux from a light source into a plurality of luminous fluxes; a second lens array having lens cells on which division luminous fluxes obtained by the lens cells of the first lens array are made incident; and an optical element configured to illuminate an image display element by superposing luminous fluxes emitted from the second lens array, at least one of the first and second lens arrays having at least two lens cells composed so that the curved surfaces of adjacent lens cells are continuous, and the two lens cells being eccentric. In the illumination optical system, the curvature radii of the at least two lens cells of the first lens array are different from each other.