An imaging optical element is comprised of a gradient index rod lens 12 of which external peripheral surface is a smooth surface without being processed by means of a flare-cut treatment and a field limitation stop member 14 mounted on the lens surface at the incidence side thereof. As for the stop member, the length t thereof is: t>1o·D/2X (where, 1o: a lens working distance, D: a rod lens diameter, X: a radius of field of view), whereby the angle of incidence of beam &thgr; at the center of the lens satisfies &thgr;