An electrolyzer structure is provided that is suitable for use in a method that produces 
hydrogen by steam 
electrolysis by feeding reducing gas to an 
anode side and feeding steam to a 
cathode side of an electrolyzer that is partitioned into the 
anode side and the 
cathode side by a diaphragm of 
solid oxide electrolyte, and feeding power to the 
anode and 
cathode of the electrolyzer. One embodiment of the present invention relates to an apparatus that produces 
hydrogen by high temperature steam 
electrolysis and that comprises an electrolyzer partitioned into an anode side and a cathode side by a 
solid oxide electrolyte diaphragm, a conduit that feeds reducing gas to the anode side of the electrolyzer, and a conduit that feeds steam to the cathode of the electrolyzer, in which a 
metal cermet stable in a 
reducing atmosphere is used as the material of the anode and the cathode. Another embodiment of the present invention relates to a method of producing 
hydrogen by high temperature steam 
electrolysis for reducing electrolysis 
voltage by feeding steam to a cathode side and feeding 
hydrocarbon-containing gas to an anode side for reaction with 
oxygen ion, the cathode side and the anode side being provided in a high temperature steam electrolytic apparatus in which an electrolyzer is partitioned into the anode side and the cathode side using a 
solid oxide electrolyte as the diaphragm, wherein offgas discharged from the anode side of the electrolytic apparatus is admixed into the 
hydrocarbon-containing gas that is fed to the anode side of the electrolytic apparatus.