PROTON-CONDUCTIVE COMPLEX OXIDE AND FUEL CELL USING SAME AS ELECTROLYTE

Provided is a complex oxide that has a space group I-43d, has a high hydrogen content, contains almost no impurity phase, exhibits almost no aluminum substitution in the structure thereof, and is suitable for proton conductivity. This complex oxide is represented by a chemical formula LiHLaZrMO(M represents Ta and/or Nb, 3.2