An imaging lens system includes a plastic lens element. The plastic lens element includes an optical effective portion and an outer ring portion. The optical axis passes through the optical effective portion. The outer ring portion surrounds the optical effective portion and includes an annular groove structure, a conical surface, a flat abutting portion and a full-circle connecting portion. The annular groove structure is tapered off. Each of the conical surface and the flat abutting portion is located closer to the optical effective portion than the annular groove structure. The flat abutting portion is in physical contact with an optical element. The full-circle connecting portion is connected to the annular groove structure and located farther away from the optical effective portion than the annular groove structure. The annular groove structure has an annular bottom end surface extending in a direction substantially perpendicular to the optical axis.