The electromotive force (EMF) of the hydrogen isotope cell with high-temperature proton conducting solid electrolytes was investigated. Stable and reproducible EMF was observed. The EMF could be attributed to the internal potential in the electrolyte and redox potentials of H-2 and D-2 gases. The former is based on the mobility differences between protons and deuterons. In addition, there would be a contribution of oxygen concentration cell to the EMF of the cell in the case of the SrCe0.95Yb0.05O3-alpha electrolyte due to its partial oxide ion conduction in the high-temperature region.