A projection optical system and that includes a relatively small number of lenses and are able to cover a wide zooming range and a projector. A 1-2 lens group which is a focus lens group is constituted with a lens which includes a single positive lens having a convex surface to the reduction side, a lens which includes a single negative meniscus lens having a convex surface to an enlargement side, and a lens which includes a single negative lens, and the 1-2 lens group is moved at the time of focusing accompanying magnification change.