A zooming optical system includes, in order from an object side along an optical axis: a first lens group having positive refractive power; a second lens group having negative refractive power; a third lens group having positive refractive power; and a fourth lens group having positive refractive power; upon zooming from a wide-angle end state to a telephoto end state, a distance between the first lens group and the second lens group is varied, a distance between the second lens group and the third lens group is varied, and a distance between the third lens group and the fourth lens group is varied; the third lens group is composed of one positive lens; and a focus from an infinite-distance object up to a near-distance object is conducted by moving the third lens group in a direction of the optical axis.