During liquid chemical cleaning treatment, leaching of buried plugs occurs from a portion where the buried plugs are exposed locally to result in increase of resistance, lowering of electric conduction yield, lowering of device yield and deterioration of reliability. In a method of manufacturing a semiconductor device by forming upper layer interconnections on buried plugs formed in an interlayer insulating film, the upper layer interconnections are formed by patterning using etching and then plasma processing using an oxygen series gas with addition of a fluorine series gas is applied to the surface of the buried plugs formed being extended out of the upper layer interconnections, before removing the resist film 19 used as an etching mask at least by the organic stripping liquid, thereby forming a protection film on the surface of the buried plugs.