PURPOSE: To prevent electrical sticking of a variety of dust, etc. and the corrosion on the surface of image sensing elements, by forming protective layers consisting of phenolic resins on at least one of the light incident end face and light exit end face of an image forming optical system using a small diameter array of a short focus.

CONSTITUTION: The end face of an image forming optical system 3 using a small image forming element array 1 of a short focus is easy to pick up dust as said surface is charged electrostatically to an extremely high potential by corona discharge. The element surface 1 is also corroded by the corrosive materials, such as nitric acid, formed by the ozone generared by corona discharge, ionized gaseous molecules, steam, etc., on account of which a considerable decrease in the performance of the optical system results. The protective layers consisting of phenolic resins formed on the three end faces of the image forming optical system are extremely hard, are highly mechanically durable, have relatively low electrical resistance, and are extremely stable to corrosive materials such as acids and alkalis. Therefore, said protective layers decrease the generation of electric lines of force having a dust collecting effect, and prevent the corrosion on the surfaces of the image forming elements occuring in exposure to the corrosive chemical seeds generated from corona electrifiers, etc.