Abstract: A method of forming an insulating film for enhancing the film and removing water and OH groups from the film efficiently. The method comprises steps of forming a second low dielectric constant insulating film having a thickness smaller than a desired thickness in a predetermined material gas atmosphere, carrying out O2 gas treatment at the same substrate temperature and pressure as those in the step of forming the second low dielectric constant insulating film, carrying out residual unreacted material removal, uncombined bonds termination, or absorbed OH group removal, without air exposure, and forming a second cap insulating film. The steps of forming the second low dielectric constant insulating film, carrying out processes such as the residual unreacted material removal, and forming the second cap insulating film are repeated until a total thickness of the second low dielectric constant insulating film and the second cap insulating film reaches a desired level.