To provide a color synthesis optical system capable of obtaining an image free of registration deviation.

This optical system comprises a liquid crystal light valve LC having plural pixels and a cross dichroic prism which color synthesizes the light beams emitted from this liquid crystal light valve LC. In such a case, the optical system is so constituted as to satisfy L×tan(Θ)≤W/2 when the length in the horizontal direction of the pixel pitch of the liquid crystal light valve LC is defined as W, the reduction length in terms of air from the exit surface of the liquid crystal light valve LC to the exit surface of the cross dichroic prism as L and the value of the relative argument when the two rays emitted from the liquid crystal light valve LC and parallel with the optical axis are made incident on the cross dichroic prism and are emitted from the cross dichroic prism as Θ (degree).