PURPOSE: To reduce the cost and to maintain a sufficient performance even though environmental conditions including temperature and humidity vary by employing many plastic lenses.

CONSTITUTION: The lens system is constituted by three lenses, i.e., a plastic concave lens 1, a plastic convex lens 2 and a glass lens 3. The two object side lenses are made of plastic so as to reduce the cost of the lens system. Moreover, the lenses 1 and 2 of the object side are made into an afocal system wide converter so that, no change is caused in the characteristic by the variation in the environmental conditions including temperature and humidity and a sufficient performance is maintained. Furthermore, since at least one of the respective surfaces of the two plastic lenses are formed into aspherical, aberration compensation is easily done and sufficient aberration compensation is made even though the image angle becomes larger by the wide converter.