Even when the refractive index of the plastic material forming a plastic lens fluctuates due to temperature change and the like, the change in the paraxial back focus amount caused by this fluctuation is canceled by spherical aberration which is generated by an aspheric surface and yields an action in a direction opposite thereto, whereby the fluctuation in refractive index in the plastic lens due to temperature change and the like is suppressed while a demand for attaining a compact size with high performances is satisfied. The second lens (L.sub.2) in the first lens group (G.sub.1) and the fourth lens (L.sub.4) in the second lens group (G.sub.2) are formed by plastic lenses, while each of the both sides of these plastic lenses is made as an aspheric surface, whereby, even when the refractive index of the plastic material fluctuates due to temperature change and the like, the change in paraxial back focus amount caused by this fluctuation is canceled by the spherical aberration which is generated by this aspheric surface and yields an action in a direction opposite thereto, so as to suppress the fluctuation in imaging position.