The reaction scheme of ammonia synthesis in the ECR plasma apparatus was investigated from both identifications of the species in the plasmas and the adsorbed species on the surface of a steel substrate placed in the plasmas. The adsorbed species were considerably different when different kinds of plasmas are used. NHx species were adsorbed on the steel substrate surface in the nitrogen-hydrogen plasma, and N-2 molecules were adsorbed in the nitrogen plasma. By the application of a negative bias potential on the substrate, the adsorption of N atom or Fe-N bond formation was identified on the steel substrate surface. When the stainless steel wall of the chamber was covered with aluminum foil, the yield of NHx radicals, which were on both the substrate and in the plasma, decreased. By exposure of the substrate, on which N-2 molecules or N atoms adsorbed, to the hydrogen plasma, N-2 and N disappeared from the steel substrate surface, forming ammonia. Moreover, the adsorption of NHx radicals disappeared when the stainless steel wall surface was covered with aluminum foil. Thus, the surface of the stainless steel wall acts as a catalyst in ammonia formation. The formation of ammonia in the nitrogen-hydrogen ECR plasma, in which the steel substrate served as the catalyst, is nor only through the dissociative adsorption of excited nitrogen molecules but also through the dissociative adsorption of nitrogen molecular ions.