Etching and protective-film deposition operations E and D are in alternation repeatedly executed on a silicon substrate carried on a platform within a processing chamber. With gas inside the processing chamber having been exhausted to pump down the chamber interior, in the etching operation E, the substrate is etched by supplying etching gas into the chamber and converting it into plasma and applying a bias potential to the platform, and in the protective-film deposition operation D, a protective film is formed on the silicon substrate by supplying protective-film deposition gas into the processing chamber and converting it into plasma. When a predetermined time prior to the close of operations E and D (time intervals indicated by reference marks Ee and De) is reached, the supply of etching or protective-film deposition gas is halted, and the exhaust flow rate of gas exhausted from the chamber is made greater than that previously.