PURPOSE: To provide a compact optical system free from interference on a sensor by making the array direction of plural luminous fluxes and the luminous flux separating direction of a crystal prism, which has the birefringence property, approximately orthogonal to each other.

CONSTITUTION: Reflected luminous fluxes 50, 60, and 70 are made incident on a crystal prism 11 having the birefringence property. The prism 11 consists of a prism 1a which has the crystal axis perpendicular to the array direction and the incidence direction of luminous fluxes 50, 60, and 70 and a prism 11b which has the crystal axis orthogonal to that of the prism 11a. The joint surface is parallel with the array direction of luminous fluxes 50, 60, and 70, and its normal is about 45° to the luminous flux incidence direction. Each of luminous fluxes 50, 60, and 70 made incident on the prism 11 has polarization directions orthogonal to each other and is separated into two luminous fluxes in the direction perpendicular to the array direction of luminous fluxes 50, 60, and 70 and is condensed on an RF sensor 13 by a condenser lens 8. Thus, inference on the IR sensor is easily avoided, and the compact optical system is constituted.