NOVELTY - A transition metal nitride comprises two layers. The first layer is made of a nitride of a stainless steel containing at least iron and chromium. The second layer is made of another nitride having a different composition from that of first layer. The first and second layers have a composition distribution in which a chromium concentration is varied from the first to second layer in a thickness direction of the layers. The second layer has a nitride deposition protruding from a base portion of the exposed surface at its exposed surface.    USE - The transition metal nitride is useful for producing fuel cell separator, fuel cell stack and fuel cell vehicle.    ADVANTAGE - The transition metal nitride is low in contact resistance and excellent in corrosion resistance. The transition metal nitride provides fuel cell separator with the same characteristics as that of the transition metal nitride. The transition metal nitride provides fuel cell separator with high performance, and can be easily produced. The transition metal nitride allows downsizing and cost reduction that may be achieved and flexibility in styling can be ensured wile increasing mileage.    DETAILED DESCRIPTION - A transition metal nitride comprises two layers. The first layer is formed of a nitride of a stainless steel containing at least iron and chromium. The second layer is formed on the first layer. The second layer has an exposed surface. The second layer is formed of another nitride having a different composition from that of the first layer. The first and second layers have a composition distribution in which a chromium concentration is varied from the first layer to the second layer in a thickness direction of the layers. The second layer has a nitride deposition protruding from a base portion of the exposed surface at its exposed surface. INDEPENDENT CLAIMS are included for:    (1) fuel cell separator comprising a base layer made of stainless steel containing iron and chromium, and a nitride layer containing a transition metal nitride;    (2) producing transition metal nitride comprising carrying out a plasma nitriding on a surface of a base material formed of a stainless steel while holding the surface at greater than or equal to25degreesC thus forming two layers;    (3) fuel cell stack comprising fuel cell separators alternatively stacked with membrane electrode assemblies; and    (4) vehicle comprising a fuel cell stack.    DESCRIPTION OF DRAWING(S) - The figure shows a perspective view of the fuel cell stack of fuel cell separator.    Air supply lines (AL)    Hydrogen supply lines (HL)    Water supply lines (WL)