Steam electrolysis is attracting much interest method to produce hydrogen and also the new energy recovery process of wasted heat energy. Up to now, Y2O3 stabilized ZrO2 (YSZ) has been used for a solid electrolyte and so the operating temperature is limited down to 1273K. This study is focused on increasing the performance of steam electrolysis by using LaGaO3 based oxide for electrolyte at intermediate temperature of 873 K, which is upper limit of the obtainable wasted heat. It was found that the formation amount of H2 is almost obeyed the Faraday law up to 1.8 V suggesting that the ionic transport number of oxide ion in LaGaO3 was kept to be 1 under the steam electrolysis condition. The electrolyzing current is improved as following order; La0.6Sr0.4CoO3<Sm0.5Sr0.5CoO3<< Ba0.6La0.4CoO3 for anode and Pt<Ni<Ni-Fe for cathode, respectively. Hydrogen production rate higher than 100 μmol ml-1 min-1 at 873 K and 300 mA cm-2 were successfully demonstrated at 1.8 V in this study. Electrolysis reaction under various reaction conditions are also presented. The H2 formation rate increased with increasing total flow rate due to the diffusion resistance.