PURPOSE: To attain to increase a nitriding speed to a large extent, to reduce operation cost and to enhance safety, by adding CO2 to a gaseous soft nitriding atmosphere comprising an N2 gas, an NH3 gas and an org. solvent decomposing gas to subject a material to be treated to gaseous soft nitridation.

CONSTITUTION: A CO2 gas is added to a gaseous soft nitriding atmosphere comprising an N2 gas, an NH3 gas and an org. solvent decomposing gas such as a methanol decomposing gas in an amount of 0.1W10vol%. By using this CO2 added gaseous soft nitriding atmosphere, a material to be treated is subjected to gaseous soft nitriding treatment within a temp. range of 450W650°C. When the CO2 gas is added to mentioned above, CO2 is reacted with a H2 gas imparting bad influence to nitriding reaction to lower H2 partial pressure and, therefore, a nitriding speed can be accerelated.