A steam electrolysis cell was constructed with a high-temperature type protonic conductor, SrZr0.9Yb0.1O3-alpha. The reduction of nitrogen oxide (NO) by hydrogen, which was produced by a steam electrolysis cell, was examined. Helium gas with 8% O-2 and 1000 ppm NO was used as a model gas for the exhaust gas from automotive engines operating under lean-burn condition. The removal efficiency of NO was measured, using steam electrolysis cells with different catalysts on the cathodes. A mixture of Pt-sponge and Sr/Al2O3 catalyst was found to be most effective for the preferred reduction of NO in the presence of excess O-2. It was suggested that the preferred reduction of NO occurred via the electrochemical reduction of NO absorbed into Sr/Al2O3, not via the chemical reduction of NO by H-2 gas.