To obtain a compact image formation optical system having high- performance by uniformalizing lengths of optical paths of color lights, respectively, in a color light separating optical system without using a relay optical or drawing around a complicated optical path, and also to obtain a projector using the same.

This image formation optical system is provide with dichroic mirrors 211, 212 which separate light from a light source 110 into red, green and blue color lights, mirrors 221, 222, 223, 224 which form mutually symmetrical optical paths by reflecting two color lights among those separated with the dichroic mirror on the same horizontal surface, and mirrors 231, 232, 233, 234 which form a protruded optical path by reflecting the last color light separated with the dichroic mirror orthogonally or parallel to the horizontal surface.