The electromotive force (EMF) of hydrogen isotope cell with a proton-conducting solid electrolyte CaZr0.90In0.10O3-alpha was investigated at 500 similar to 900 degrees C. This kind of cell (H-2/D-2) gave steady and reproducible EMF for which the D-2 side electrode was positive. The EMF could be explained by the two factors, i.e., the difference in mobilities of proton and deuteron in the electrolyte and the electrode reactions of H-2 and D-2 gases. The EMF of the cell for mixed isotope gases was nearly linear with the composition of hydrogen isotopes with a slight deviation and was consistent with such a mechanism of EMF of hydrogen isotope cell.