To constitute an illuminating optical device for a light valve type projector device so that it may occupy a short space in terms of a distance.

The illuminating optical system is provided with a light source 21, a 1st multi-lens array 24 for condensing light emitted from the light source 21, a 2nd multi-lens array 25 on which the light condensed through the 1st multi lens array 24 is made incident, and a light condensing lens 22 for condensing the light from the 2nd multi-lens array 25, and as for the system, a 1st reflection surface 1 for reflecting the light from the light condensing lens 22 and a 2nd reflection surface 2 for reflecting the light from the 1st reflection surface 1 are arranged so that a luminous flux P1 from the light condensing lens 22, a luminous flux P2 from the 1st reflection surface 1 and a luminous flux P3 from the 2nd reflection surface 2 may mutually cross.