PURPOSE: To apply an optical coating having good wear and adhesive characteristics on a workpiece surface by introducing an inert gas of a high mol.wt. into a chamber and ionizing this gas to effect ion bombardment on the workpiece surface, thereby opening the chemical bonds of the workpiece surface.

CONSTITUTION: The inside of the chamber 18 is evacuated and the inert gas of the relatively high mol.wt., such as Ar is filled therein. Negative potential is applied on a ring electrode 26 to ionize the gas and the ions are migrated in an excitation state to effect the passive bombardment of the workpiece 21 of the synthetic resin. Consequently, the oxide, radicals, contaminants, etc., remaining on the workpiece 21 are removed to the ionized atmosphere. Further, the chemical bonds on the substrate surface of the workpiece 21 are opened without the fissure of the lower layer bond structure of the workpiece 21 by the ion bombardment of the workpiece 21. The pressure in the chamber 18 is then regulated to start the vapor deposition coating.