NOVELTY - The transition metal nitride obtained by nitriding a base aterial including an austenitic stainless steel having a chromium concentration of greater than or equal to 25%, comprises a first layer formed continuously on a base layer formed by the base material, having a stacked crystal structure of a nano-level including a nitride having a cubic crystal structure, and a nitride having a hexagonal crystal structure, and a second layer formed continuously on the first layer, including a nitride having hexagonal crystal structures of chromium nitrides and a cubic crystal structure.    USE - The transition metal nitride is useful in separator for fuel cells (claimed).    ADVANTAGE - The transition metal nitride has excellent conductivity and corrosion resistance.    DETAILED DESCRIPTION - The transition metal nitride obtained by nitriding a base aterial including an austenitic stainless steel having a chromium concentration of greater than or equal to 25%, comprises a first layer formed continuously on a base layer formed by the base material, having a stacked crystal structure of a nano-level including a nitride having a cubic crystal structure of M4N type, and a nitride having a hexagonal crystal structure of M2-3N type, and a second layer formed continuously on the first layer, including a nitride having hexagonal crystal structures of chromium nitrides (Cr2N, CrN) and M2-3N type, and a cubic crystal structure of M4N type, and formed as a surface-nitriding-processed portion of the base material continuously in a depth direction from a surface of the base material, where M is a transition metal atoms of iron, chromium, nickel and molybdenum. The first layer has a composition including M (66.8-80 at%), and N (20 to 33.3 at%) and the second layer has a composition including M (50 to 75 at%) and N (25 to 50 at%). The second layer contains a layered form of nitride within a thickness of 3-30 nm. The second layer has a precipitate projecting from a surface portion of the second layer, and the precipitate has a height of 10-90 nm relative to the surface portion of the second layer. An amount of precipitates projecting from the surface portion of the second layer is greater than or equal to 5% in area ratio. The metal nitride comprises 800 ore more precipitates of equivalent circle diameters of greater than or equal to 40 nm per area of 100 mu m2 projecting from the surface portion of the second layer. An amount of precipitates projecting from the surface portion of the second layer after a dissolution treatment by acid of the transition metal nitride is greater than or equal to 5% in area ratio. The second layer has a composition including chromium (10-30 at%) and N (20-40 at%) having chromium Cr and N incrassated in part, at a depth of less than or equal to 10 nm from the surface of the base material. INDEPENDENT CLAIMS are included for:    (1) a separator for fuel cells;    (2) a method of manufacturing a transition metal nitride to be formed by nitriding a base material;    (3) a method of manufacturing a separator for fuel cells;    (4) a fuel cell stack; and    (5) a fuel cell vehicle.    DESCRIPTION OF DRAWING(S) - The drawing shows the separator for fuel cells and a second nitrided layer of the separator.    Base material (10)    Surface portion (10a)    Nitrided layer (11)    First nitrided layer (11b)    Base layer. (12)