An illuminating optical system for a projector including a light source and a plurality of display panels. The optical system includes a first integrator, which includes a plurality of lens cells that divide a luminous flux emitted by the light source into a plurality of partial luminous fluxes and that condense the respective partial luminous fluxes, a second integrator, which includes a plurality of lens cells on which the respective partial luminous fluxes are incident, a polarization converting element, which uniformizes polarizing directions of the luminous fluxes having passed through the respective lens cells of the second integrator, and a color separating unit, which separates each of the luminous fluxes from the polarization converting element into a plurality of color lights of different wavelengths. A field lens and a condenser lens are arranged between the polarization converting element and the color separating unit. The field lens superimposes, on the plurality of display panels, the luminous fluxes having passed through the respective lens cells of the first integrator.