A low relative permittivity SiO.sub.x film excellent in heat resistance without using an alkali metal, fluorine, etc., a method for modifying an SiO.sub.x film to accomplish a further reduction of the relative permittivity of the low relative permittivity SiO.sub.x film and further to increase the insulating property, a highly reliable semiconductor device free from crack or peeling of the film by employing the low relative permittivity SiO.sub.x film as an interlayer insulating film for metal wirings, are provided. The low relative permittivity film is characterized in that it is made of a porous material, the major constituent of which is SiO.sub.x (where 1.8.gtoreq.X.gtoreq.1.0), and the relative permittivity at 1 MHz is at most 2.3.