Steam electrolysis was studied by using a new oxide cathode of Ce0.6Mn0.3Fe0.1O2-δ (CMF). It was found that this cathode showed almost the same performance as a conventional Ni-based cathode without H2 flow, and H2 generation rate obeyed the Faraday's law at 900-700°C. By XRD measurements after operation, CMF phase was hardly changed after steam electrolysis. A long-term durability test was operated, and it revealed that CMF cathode showed good stability over 90 h at 800°C. The impedance spectra were measured under applied DC bias potentials. These revealed that polarization resistance and ohmic resistance decreased with increasing DC bias potential. This decrease of ohmic resistance was related to the increase in electronic conductivity of CMF. CMF was n-type semiconductor and the electronic conductivity of CMF increased with decreasing oxygen partial pressure.