Protonic ceramic fuel cells (PCFCs) are clean power generation devices that do not emit carbon dioxide, and generate electricity when protons (hydrogen ions) pass from the fuel electrode through the electrolyte to form water vapor at the air electrode. As a fuel cell, PCFC is expected to be a highly efficient fuel cell because it can operate at medium to low temperatures of around 600°C and no fuel dilution occurs. However, proton-conducting ceramics can conduct not only protons but also a small amount of holes, which can cause electron leakage and lower the power generation efficiency. Therefore, it is very important to understand the ionic and hole conductivity in a fuel cell. To evaluate the electronic leakage of PCFC, the amount of proton current flowed in PCFC can be measured by an apparatus developed based on the electromotive force of an electrochemical cell using 10 mol% In-doped CaZrO3 (hydrogen sensor) and Y stabilized zirconia (oxygen sensor). Water vapor pressure on cathode was change when hydrogen ion passes thorough PCFC. The current of hydrogen ion can be determined by monitoring water vapor pressure. We measured the current of hydrogen flowing through protonic ceramic fuel cell using ytterbium-doped barium zirconate as electrolyte. Hydrogen ion and hole were found to leak at OCV at 700°C. Then electronic leakage was estimated form the current of hydrogen ion and external current. The electronic leakage was found to be suppressed with increasing electrolyte film thickness. On the other hand, no leakage of hydrogen and holes was observed at 500°C.