The activity test of commercial catalysts of steam-hydrocarbon reforming was carried out under the wide range of working conditions. (T=650∼850℃; S. V. =600∼1,200 hr^-1; steam to carbon ratio=1.17) An activity test method has been developed through the parallel-triple-calumn gas chromatographic technique. The following results were obtained: (1) Hydrocarbons are scarcely found in the outlet gas except methane, and the gas composition could be approximately predicted by assuming the chemical equilibrium at the reaction gas temperature, among H₂, CO, CO₂, CH₄, and H₂O. (2) It is difficult to compare the activity of catalysts by any direct comparisons of kinetic data at the high conversion level of reactants, while a conventional method proposed by Inoue et al is eligible. (3) It was concluded that the activity of commercial catalysts A, B, and C are approximately same. (4) Steam to carbon ratio to prevent the carbon deposit was examined experimentally and theoretically.