An image forming optical system includes a first image forming unit configured to form an image of a first surface onto a second surface, a second image forming unit including at least a portion of the first image forming unit and configured to form an image of the second surface onto a third surface, and a deflection unit configured to deflect light from the second surface toward the third surface. The numerical aperture of the first image forming unit on the side toward the second surface is larger than the numerical aperture of the second image forming unit on the side toward the second surface.