An imaging optical system at least includes a variable magnification optical system that includes, in order from the object side, a positive, first lens unit, a negative, second lens unit, a positive, third lens unit, a positive, fourth lens unit, and an aperture stop arranged between the third lens unit and the fourth lens unit, to change magnification by changing a distance between the first lens unit and the second lens unit, a distance between the second lens unit and the third lens unit, and a distance between the third lens unit and the fourth lens unit. The imaging optical system changes the magnification while keeping a constant object-to-image distance, and satisfies the following conditions in at least one magnification state: |En|/L>0.4 |Ex|/|L/β|>0.4 where En is a distance from an object-side, first lens surface of the variable magnification optical system Z to the entrance pupil of the imaging optical system, L is the object-to-image distance of the imaging optical system, Ex is a distance from the image-side, last lens surface of the variable magnification optical system Z to the exit pupil of the imaging optical system, and β is a magnification of the entire imaging optical system.