To improve toughness of a nitrided layer, continuously reduce hardness of the nitrided layer to improve adhesion to a base steel layer, and improve various resistances, by using marageing steel or the like for material of a metallic cutter, and forming a nitrogen diffused layer on the surface of the cutter by a bright nitrogen diffusion method.

In a metallic cutter, a material to be cut placed on a lower cutter is pressed by an upper cutter 2 to be cut off. The uppermost surface 2A of the upper cutter 2 contacted with the material to be cut is formed with a radical nitrided layer 2B of thickness, for example, of about 30-300 μm. Because the upper cutter 2 and the lower cutter are metallic cutter, they are desirable to be treated by radical nitriding on heattreated marageing steel. For forming a radical nitrided layer, the upper cutter 2 made of degreased and washed marageing steel is placed on the negative electrode in a vacuum furnace and after evacuating the vacuum furnace, it is heated in the mixed gas atmosphere to impress dc voltage between it and the positive electrode, and the gas is ionized by glow discharge to diffuse nitrogen on the surface of the cutter.