To provide a projection optical system which has a small ratio of lens diameter to the maximum image height on a reduction side and aberrations are excellently compensated.

The system is composed, in sequence from the enlarging side, of a first lens group (Gr1) having a negative power, a second lens group (Gr2) having a positive power, and a third lens group (Gr3) having a positive power, the third lens group (Gr3) is composed, in sequence from the enlarging side, of a negative lens (GN) having a concave surface on the reduction side and a positive lens (GP), and the condition 0.5<T/Hmax<3.0 is satisfied, where T stands for the distance between the negative and the positive lenses (GN, GP) and Hmax stands for the maximum height of an image on the reduction side.