The present invention is a processing method for a metal component by using a processing furnace. The method includes the steps of: introducing an activation atmospheric gas into the processing furnace; heating the activation atmospheric gas in the processing furnace to a first temperature; introducing a nitriding atmospheric gas or a nitrocarburizing atmospheric gas into the processing furnace; and heating the nitriding atmospheric gas or the nitrocarburizing atmospheric gas in the processing furnace to a second temperature. The activation atmospheric gas is introduced into the processing furnace through a pipe for introducing the activation atmospheric gas. A liquid organic solvent is introduced intermittently a plurality of times into the pipe for introducing the activation atmospheric gas which is under a state wherein the activation atmosphere gas continues to be introduced.