In the film forming method of the present invention, a substrate is first received in a processing vessel. Then, a film-forming gas including an organic silicon and an additive gas including a paraffin hydrocarbon gas and/or a hydrogen gas are introduced into the processing vessel, and the film-forming gas and the additive gas are then converted into plasma. In this manner, a carbon-hydrogen-added silicon oxide film (SiCOH film) is formed on the substrate. Alternatively, the film-forming gas is first introduced into the processing vessel containing the substrate and is then converted into plasma to form the SiCOH film on the substrate. Subsequently, the additive gas is introduced into the processing vessel and is then converted into plasma to post-treat the substrate having the SiCOH film formed thereon.