A plasma CVD device having a chamber, an upper electrode provided in the chamber, an under electrode provided in the chamber to be opposite to the upper electrode and to mount a sample thereon, and a plurality of power sources having a different frequency connected to the upper electrode. Gas is introduced into the chamber of the plasma CVD device, the gas contains at least an organic silicon compound, CF.sub.4 and O.sub.2, and has an element ratio (F/Si) of silicon (Si), constituting the organic silicon compound, to fluorine (F), constituting CF.sub.4, to be set to 15 or more. Si(OC.sub.2 H.sub.5).sub.4 or Si(OCH.sub.3).sub.4 is used as an organic silicon compound.