A variable magnification optical system for forming an optical image of an object on the light-receiving surface of an image sensor with variable magnification includes: from an object side, a first lens unit having a positive optical power, a second lens unit having a negative optical power, a third lens unit having a positive optical power, a fourth lens unit having a positive optical power, and a fifth lens unit. In magnification variation from the wide-angle end to the telephoto end, the first lens unit, the third lens unit, and the fifth lens unit are kept in fixed positions whereas the second lens unit and the fourth lens unit move. The third lens unit includes, from the object side, a positive lens element having a biconvex form, and a negative lens element having a form concave to an image side. The negative lens element has an aspherical surface.