To provide a film forming method and a film forming device where a silica based film can be formed under ordinary pressure while preventing the increase of its dielectric constant caused by the generation of ozone.

This film forming method and film forming device have a means for irradiating a composition for forming a silica based film with ultraviolet rays through a shielding member shielding at least a part of light with a wavelength of 200 nm while heating the same. By the provision of this means, a silica based film can be formed under ordinary pressure while preventing the increase of its dielectric constant caused by the generation of ozone.