In a formation method of a porous insulating film by supplying at least organosiloxane and an inert gas to a reaction chamber and forming an insulating film by a plasma vapor deposition method, a partial pressure of the organosiloxane in the reaction chamber is changed by varying a volume ratio of the organosiloxane and the inert gas to be supplied during deposition. Thus, the dielectric constant of the insulating film in the semiconductor device is reduced while the adhesion of the insulating film with other materials is improved. It is desirable that the organosiloxane be cyclic organosiloxane including at least silicon, oxygen, carbon, and hydrogen, and that the total pressure of the reaction chamber be constant during deposition.