An electrode coated with a polymeric material at a percentage of covering of 50 to 100 % is used as the electrode facing that side of a continuous substrate on which a film is to be formed by plasma polymerization. Plasma polymerization is conducted under the conditions of an operating pressure of 10-3 to 1 Torr to form a film on the substrate. Consequently, the method produces an excellent effect of remarkably suppressing abnormal discharge during the film formation by plasma polymerization, thus making it possible to stably form a film by plasma polymerization over long and thus improve the yield of the product. The film formed by the plasma polymerization has extremely good properties.