A projection optical system substantially consists of a first optical system composed of a plurality of lens groups and a second optical system composed of one reflection mirror having a convex aspherical surface arranged in this order from the reduction side, in which all optical surfaces constituting the first and second optical systems are formed so as to have rotationally symmetrical shapes around one common axis, the projection optical system is configured such that focus adjustment is performed by individually moving two lens groups in the first optical system along the common axis, and the lens disposed on the most magnification side in the reduction side lens group of the two lens groups is a lens having a convex surface on the magnification side, thereby magnifying and projecting an image formed on a conjugate plane on the reduction side to a conjugate plane on the magnification side.