A plasma processing method for use in device isolation by shallow trench isolation in which an insulating film is embedded in a trench formed in silicon and the insulating film is planarized to form a device isolation film, the method includes a plasma nitriding the silicon of an inner wall surface of the trench by using a plasma before embedding the insulating film in the trench. The plasma nitriding is performed by using a plasma of a processing gas containing a nitrogen-containing gas under conditions in which a processing pressure ranges from 1.3 Pa to 187 Pa and a ratio of a volumetric flow rate of the nitrogen-containing gas to a volumetric flow rate of the entire processing gas ranges from 1% to 80% such that a silicon nitride film is formed on the inner wall surface of the trench to have a thickness of 1 to 10 nm.