An electrolytic water removal device using a solid polymer electrolyte membrane has been developed for controlling the humidity in the atmosphere of electrical control boxes. The device consists of a solid polymer electrolyte and thin film electrodes. The anode side of the device shows a dehumidifying effect while the cathode side shows a humidifying effect. This paper reports the evaluation and separation of the anode and cathode potentials by installing a gas port as a reversible hydrogen reference electrode attached to the water removal device. The polarization curves show that the anode potential remained stable at 2 V vs RHE, while the cathode potential changed significantly.