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.