To easily obtain a prescribed back focus by appropriately setting a lens constitution.

The projection optical system is constituted of a retrofocusing 1st group L1 which is provided with plural lenses from a 1st conjugate point at a long distance in order and whose refractive index is positive, and a 2nd group L2 constituted of one lens, or a positive laminated lens L21. Each element is set so that an off-axis main beam may cross the optical axis between the 1st group L1 and the 2nd group L21. In this case, provided that a lens distance between the 1st lens L11 and the 2nd lens L12 in the 1st group counted from the 1st conjugate point is expressed by D2, a distance front the lens face apex on the 1st conjugate point side of the lens L21 to a 2nd conjugate point at a short distance is expressed by D2f, and the focal distance of the whole system is expressed by (f), the following conditions are satisfied; 0.3<D2/f<1.2 and 0.2<D2f/f<1.4. Thus, the projection optical system whose image side is telecentric, where asymmetrical aberration is satisfactorily corrected, and which is suitable to a projector is obtained.