The change of the primary energy source from fossil fuels to the so-called renewable energies, such as solar and wind energy, is a major and necessary trend. The use of these energies needs efficient methods of energy storing and restoring, and hydrogen can serve for an effective conversion between the electricity and the chemical energy. This paper describes the ion-conducting materials applicable to the so-called “hydrogen energy system”. Since hydrogen works as an energy medium in this system, hydrogen-ion-transporting materials are beneficial. Proton-conducting oxides will work for this purpose and will be introduced here, viewing from material designing and working mechanism. Application of the proton-conducting oxide materials to water electrolysis is another topic, and how the materials work in principle for conversion between electricity and chemical energy and how their performances could/can be improved will be described.