A projection optical system is for an image projection device. The projection optical system includes a whole lens system of seven to nine lenses and an aperture stop. The seven to nine lenses are all single lenses. One negative lens and one or two positive lenses are located on a magnifying side with respect to the aperture stop. Two negative lenses and three or four positive lenses are located on a reducing side with respect to the aperture stop. A lens adjacent to the magnifying side of the aperture stop and a lens adjacent to the reducing side of the aperture stop are both a negative lens.