The application of a LaGaO₃ based electrolyte for steam electrolysis was studied and it was found that the H₂ formation rate obeys the Faraday's law suggesting unity of oxide ion conductivity in LaGaO₃ perovskite oxide under an electrolysis condition up to 2.0 V applied potential. Among the examined metal catalysts, nickel shows the smallest cathodic overpotential and the addition of Fe to Ni is highly effective for increasing the H₂ formation rate in the steam electrolysis at 873 K. The highest H₂ formation rate is obtained at the composition of Ni:Fe = 9:1. The improved cathodic performance results from the high dispersion state of Ni particles. Impedance measurement suggests that the diffusion resistance is the dominant overpotential in cathodic reaction of the steam electrolysis. Comparing the cathodic operation mode in a fuel cell, gas and surface chemical diffusion in the electrode catalyst tends to be the rate-determining step in the cathodic operation mode in steam electrolysis because of a large molecular size, chemical stability, and molecular weight of water. Comparing the cathodic operation mode in a fuel cell, gas diffusion in the electrode catalyst tends to be the rate-determining step because of a large molecular size and molecular weight of water. Increasing the steam partial pressure is effective for increasing H₂ formation rate because of the improved gas diffusion. Decreasing the thickness of a LaGaO₃ based electrolyte is also effective for increasing the H₂ formation rate. The H₂ formation rate reached a value as high as 180 μmol cm⁻² min at 0.2 mm thickness and 873 K.