The atomic hydrogen permeability, J(H), in fluorite-type (CeO2)(0.85)(CaO)(0.15) was measured to confirm the influence of CaO concentration in CeO2 at 1050-1800 K. The decrease of J(H) with reducing temperature and the proportionality of log J(H) to inverse temperature are suggested. The estimated activation energies are 1.0(5) +/- 0.01, 1.3(5) +/- 0.02 and 0.2(2) +/- 0.04 eV at 1800-1400, 1400-1250 and 1250-1050 K, respectively. The J(H) of (CeO2)(1 - X)(CaO)(X) decreases with increasing X and proportional to X-5/2 at 1800-1300 K under H-2-H2O atmosphere. H-2 is decomposed to 2H(+) and 2e(-) on the surface of the specimen and they permeate it. They hop between oxygen ions and cerium ions, respectively. It would be difficult for H+ to cross over Vo. which was produced by CaO, then J(H) decreased with increasing X. (CeO2)(1 - X)(CaO)(X) would be the electron-proton mixed conductor under H-2-H2O atmosphere at high temperatures.