The present invention relates to a method of lowering dielectric constant of an insulating film including Si, O and CH formed by a chemical vapor deposition process. A process gas containing hydrogen atoms is supplied into a reaction vessel. A microwave is introduced into the reaction vessel to supply a uniform electromagnetic wave, thereby a plasma containing a hydrogen radical is generated in the reaction vessel. The structure of the insulating film is modified by the hydrogen radical contained in the plasma irradiated to the insulating film, lowering the dielectric constant of the film. The microwave is supplied into the reaction vessel through a radial-slot antenna.