A projection optical system includes a first lens group having negative power, a second lens group having positive power and disposed at a reduction side of the first lens group, and a third lens group having positive power and disposed at the reduction side of the second lens group. The first lens group includes at least three negative lenses. The second lens group includes at least one positive lens. The third lens group includes a plurality of positive lenses. The at least three negative lenses of the first lens group are each a single lens. The third lens group includes a jointed lens including at least one of the plurality of positive lenses. NA is a reduction-side numerical aperture, Dst is a distortion aberration at a reduction-side maximum image height, and the projection optical system satisfies Conditional Expressions (1) and (2), 0.2