A steam electrolysis cell was constructed with a proton conductor SrZr0.9Yb0.1O3-alpha as an electrolyte. The steam electrolysis cell efficiently reduced nitrogen oxide (NO) on the cathode, using hydrogen produced by steam electrolysis as a reducing agent at around 460degreesC. When a Pt/Ba/Al2O3 catalyst was placed on the cathode, NO was reduced even under an O-2-rich atmosphere. An electrolysis cell was also constructed with an oxide-ion conductor 8 Y-ZrO2 as an electrolyte. The cell could reduce NO on the cathode, not only electrochemically electrolyzing NO directly but also chemically using hydrogen produced by steam electrolysis. However, the cell with the oxide-ion conductor could not reduce NO in an atmosphere containing O-2.