A substrate to be coated with a thin film is placed inside a vacuum chamber, an ECR plasma is generated and introduced into the vacuum chamber by means of a specified magnetic field generated inside the vacuum chamber and a reaction gas, as well as an inert gas, is introduced into the vacuum chamber while a negative DC voltage superposed with a high-frequency pulse with frequency 25-250 kHz is applied to the substrate by a voltage applying device such that the voltage of the substrate reaches a positive value instantaneously. The frequency of the superposed pulse is selected by using an ammeter to determine an optimum frequency for minimizing the load current of the voltage-applying circuit.