NOVELTY - A process gas is introduced into a processing container (1) and nitrogen plasma (I) is produced. An oxide film on to-be-processed component (W) is nitrided by plasma (I), after which the component is not arranged in container, or dummy substrate is arranged. The process gas is introduced and plasma (II) is produced. Nitrogen plasma seasoning is carried out by removing oxygen in the container using plasma (II). After seasoning, the process gas is introduced and nitrogen plasma (III) is produced. The oxide film is then nitrided using plasma (III).    USE - Plasma nitridation of substrates e.g. wafers used in manufacture of semiconductor device e.g. flat panel displays, LCDs, and solar cells.    ADVANTAGE - The method prevents generation of particles even when changing the process conditions of plasma nitriding treatment within the same processing container. The oxygen amount in the processing container is suppressed.    DETAILED DESCRIPTION - A process gas containing a noble gas and nitrogen gas is introduced into the processing container of a plasma processing apparatus (100) in a nitrogen gas/noble gas flow ratio of 0.01-0.2, and nitrogen plasma (I) is produced under conditions (I). An oxide film on the sheet(s) of to-be-processed component is nitrided by plasma (I), after which the to-be-processed component is not arranged in the container, or a dummy substrate is arranged. A process gas is introduced into the container in a nitrogen gas/noble gas flow ratio of 0.2-1, and nitrogen plasma (II) is produced under conditions (II). Nitrogen plasma seasoning is carried out by removing oxygen in the container using plasma (II). After seasoning, the process gas is introduced into the container in a nitrogen gas/noble gas flow ratio of 0.2-1, and nitrogen plasma (III) is produced. The oxide film is then nitrided using plasma (III) under conditions (III).    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic sectional view of the structure of plasma nitriding treatment apparatus. (Drawing includes non-English language text)    Processing container (1)    Mounting base (2)    Heater (5)    Plasma processing apparatus (100)    To-be-processed component (W)