A method for fabricating a semiconductor device utilizing the step of forming a first insulating film of a porous material over a substrate; the step of forming on the first insulating film a second insulating film containing a silicon compound containing Si—CH3 bonds by 30-90%, and the step of irradiating UV radiation with the second insulating film formed on the first insulating film to cure the first insulating film. Thus, UV radiation having the wavelength which eliminates CH3 groups is sufficiently absorbed by the second insulating film, whereby the first insulating film is highly strengthened with priority by the UV cure, and the first insulating film can have the film density increased without having the dielectric constant increased.