A conductive thin film is deposited on the surface of plastic film moving at high velocity by ion-plating using a pressure-gradient discharge. The moving velocity of the substrate film is as high as 8 to 30m/min. A conductive, thin film composed of indium oxide, tin oxide, TTD and similar substances is deposited on the plastic film, at a pressure of 1.times.10.sup.-5 to 1.times.10.sup.-1 Torr and a discharge voltage of 50 to 100 V.