NOVELTY - Fuel cell comprises: cathode; electrolyte membrane; and anode. The electrolyte membrane is sandwiched between the cathode and the anode. The cathode is formed of a first proton conductor is substituted metal oxide compounds (I) and a mixed ionic electronic conductor. The electrolyte membrane is formed of a second proton conductor which is substituted metal oxide compounds (II).    USE - The fuel cell is useful: for generating electric power, and in electrolysis device which is useful for generating hydrogen (claimed); and as electrolysis device.    ADVANTAGE - The fuel cell improves electric power generation efficiency.    DETAILED DESCRIPTION - Fuel cell comprises: cathode; electrolyte membrane; and anode. The electrolyte membrane is sandwiched between the cathode and the anode. The cathode is formed of a first proton conductor is substituted metal oxide compounds of formula (I) and a mixed ionic electronic conductor. The electrolyte membrane is formed of a second proton conductor which is substituted metal oxide compounds of formula (II).    A, A1b = divalent metal;    B1, B1b = tetravalent metal;    M = trivalent metal;    2(3-x) = 2a+4b+3m or 2a+4b+3m1;    b+m, b+m1 = 1; and    x = (3-2a-b)/2, where m is greater than or equal to m1.    INDEPENDENT CLAIMS are also included for:    (1) generating electric power using fuel cell, comprising preparing the fuel cell, and supplying oxygen and fuel to the cathode and the anode, respectively, to generate electric power, while a temperature of the fuel cell is maintained at 300-1100 degrees C;    (2) electrolysis device comprising cathode, electrolyte membrane and anode, where the electrolyte membrane is sandwiched between the cathode and the anode, and the anode is formed of the first proton conductor and mixed ionic electronic conductor; and    (3) generating hydrogen by electrolyzing water or an organic substance, comprising (a) preparing the electrolysis device, (b) supplying the water or the organic substance which is in contact with the anode and the cathode and (c) electrolyzing the water or the organic substance by applying a potential difference between the anode and the cathode to generate hydrogen on the cathode.