Disclosed is a film-forming method, comprising supplying into a plasma processing chamber at least three kinds of gases including a silicon compound gas, an oxidizing gas, and a rare gas, the percentage of the partial pressure of the rare gas (Pr) based on the total pressure being not smaller than 85%, i.e., 85%.ltoreq.Pr<100%, and generating a plasma within the plasma processing chamber so as to form a film of silicon oxide on a substrate to be processed.