A film forming method includes: forming a thin unit film on a target substrate by supplying processing gases sequentially and intermittently into a processing space, where the target substrate is placed, in a processing chamber of a film forming apparatus while purging the processing gases with a purge gas constantly supplied into the processing space; and repeating the forming of the thin unit film to form a film having a predetermined thickness on the target substrate. A flow rate of the purge gas supplied into the processing space is set such that the film is formed in a film forming mode in which the thin unit film is formed, irrespective of a pressure in the processing chamber.