An imaging optical system consists of, in order from its object side, a front lens group of negative refracting power, and a rear lens group of positive refracting power. A lens component is defined by a lens body having only two surfaces: an object side surface and an image side surface in contact with air on an optical axis. The front lens group comprises, in order from its object side, a first lens component concave on its image side and having negative refracting power, and a second lens component concave on its image side and having negative refracting power. The rear lens group comprises, in order from its object side, a third lens component convex on its object side and having positive refracting power, and a fourth lens component having positive refracting power. The sum of the total number of lens components in the front lens group and the total number of lens components in the rear lens group is 4. The third component comprises a plastic positive lens that includes an aspheric surface and satisfies the following conditions (1-1) and (1-2): 19