This invention relates to improvements made on the method of nitriding disclosed in the Japanese Pat. No. 776055 (Japanese Patent Publication No. 49-41023) of the same inventor. In this method, a salt-bath including titanium and/or zirconium as catalyst is used, and the electrolysis is made at relatively low temperature of less than 500.degree. C. Although nitrided case of considerably deep and high hardness is obtainable by this method, these characteristics are still confined to some extent, and besides that the method is only applicable to the low carbon, or low alloy-low carbon steels.