In the in situ cyclic voltammetry that we have developed, the anode potential was shifted, and hysteresis of the anode potential appeared during the scanning of the cathode potential when the hydrogen partial pressure was decreased to ca. 5% at the anode. It was concluded that proton deficiency was responsible for the hysteresis at the anode. Further, at this hydrogen pressure in real single-cell operation, the cathode potential (iR -free) deviated from Nernst’s law, which showed the interaction between the anode potential and the cathode potential. This means that anode polarization influenced cathode polarization under a low hydrogen partial pressure where the proton deficiency occurred.