A method is disclosed which concerns the treatment for imparting dirtproofness to a plastic lens provided in advance on the surface thereof with an antireflection film formed by vacuum evaporation of an inorganic substance. This method forms a dirtproofing film on the antireflection film by mixing a polyfluoroalkyl silane-containing coating liquid and trichlorotrifluoroethane and boiling the resultant mixture, dip coating the plastic lens in the boiled mixture, then treating the dip coated plastic lens with the vapor of fluoroethane, subsequently drying the wet plastic lens with hot air, and optionally immersing the dried plastic lens in water.