An image forming optical system includes, comprises, in order from the object side toward the image side, a first lens unit with positive refracting power, a second lens unit with positive refracting power, a third lens unit with negative refracting power, a fourth lens unit with positive refracting power, and an aperture stop interposed between the third lens unit and the fourth lens unit. The image forming optical system has a variable magnification optical system in which spacings between these lens units are changed to vary the imaging magnification, changes the imaging magnification while constantly keeping the object-to-image distance of the image forming optical system, and in at least one variable magnification state where the imaging magnification is changed, satisfies the following conditions: |En|/L>0.4 |Ex|/|L/&bgr;|>0.4 where En is a distance from a first lens surface on the object side of the variable magnification optical system to the entrance pupil of the image forming optical system, L is the object-to-image distance of the image forming optical system, Ex is a distance from the last lens surface on the image side of the variable magnification optical system to the exit pupil of the image forming optical system, and &bgr; is the magnification of the whole of the image forming optical system.