A method of manufacturing a semiconductor device according to the present invention involves forming two layers of silicon nitride films as an insulating film by reacting a nitrogen containing gas with dichlorosilane to form one silicon nitrogen film, and reacting the nitrogen containing gas with a compound composed of silicon and chlorine to form the other silicon nitride film. One silicon nitride film excels in the leak current characteristic, while the other silicon nitride film is deposited faster than the one silicon nitride film, resulting in improved insulating properties of the silicon nitride films as well as a higher throughput in the formation of the simulating film.