To modify the subject film so that the polymeric film can be opacified and the writing quality, etc., can be improved by carrying out a sputter etching treatment of the surface of the polymeric film.

The surface of a polymeric film is subjected to a sputter etching treatment in oxygen under 5-100 Pa gas pressure by vacuum glow discharge sputtering. The treatment is preferably carried out on one or both surfaces at 0.2-0.8 W/cm2 power density. For example, the polymeric film is preferably placed on an electrode or the close vicinity to the side of a cathode of opposite electrodes of a flat plate or a curved surface placed at ≥0.5 cm interval so that the discharge can be performed and a high-frequency voltage at several tens of kHz or above for an optional time is applied by applying several hundreds to several thousands of V thereto to perform the discharge treatment and sputter etching.