A variable-power optical system includes, in order from an object side thereof: a first lens group with a negative optical power; a second lens group with a positive optical power, moving when the power of the variable-power optical system varies; and a third lens group with a positive or negative optical power. The first lens group includes two or more lenses including one positive lens and one negative lens which include at least two or more lenses made of a plastic material. The variable-power optical system satisfies the predetermined conditional expressions relating to is a focal length of the first lens, a focal length of the second lens, a composite focal length of the total optical system, and an image-forming magnification of the second lens group at a telephoto end.