The invention provides an imaging optical system comprising, in order from an object side to an image side thereof, a first lens group of positive refracting power, a second lens group having positive or negative refracting power and a third lens group of positive refracting power. The optical system includes an aperture stop located between an object-side surface in the first lens group and an object-side surface in the second lens group. The lens on the most object side of the first lens group and the lens on the most image side of the third lens group remain constantly fixed, and upon focusing from a far distance to a near distance, only the second lens group moves axially. The third lens group comprises a front lens subgroup of positive refracting power located on the object side and a rear lens subgroup of negative refracting power located on the image side, with the largest axial air separation in the third lens group interposed between them.