A plasma nitriding method includes performing a high nitrogen-dose plasma nitriding process on an object having an oxide film by introducing a processing gas containing a nitrogen gas into a processing chamber of a plasma processing apparatus and generating a plasma containing a high nitrogen dose; and performing a low nitrogen-dose plasma nitriding process on the object by generating a plasma containing a low nitrogen dose. After the performing the high nitrogen-dose plasma nitriding process is completed, a plasma seasoning process is performed in the chamber by generating a nitrogen plasma containing a trace amount of oxygen by introducing a rare gas, a nitrogen gas and an oxygen gas into the chamber and setting a pressure in the chamber in a range from about 532 Pa to 833 Pa and a volume flow rate ratio of the oxygen gas in all the gases in a range from about 1.5% to 5%.