A variable-magnification optical system has a plurality of lens groups that image a light beam from the object side on an image sensor, an optical aperture stop that intercepts part of the light beam heading for the image sensor, and a reflective mirror that changes the optical axis of the light beam of which part has been intercepted by the optical aperture stop. The plurality of lens groups include at least lens groups arranged in a positive-negative-positive-positive optical power arrangement, and at least two of the lens groups are moved for zooming from the wide-angle end to the telephoto end. The variable-magnification optical system fulfills a prescribed conditional formula.