We proposed switching to a "diaphragm type" sensor in which the circumference of the measurement electrode is filled with an electrolytic solution, _and verified that the hypochlorous acid concentration can be measured stably without injecting saline solution. [Reason] At the electrode part of the sensor, the concentration is measured by observing the current flowing by "electrolysis". In the "membrane type", the electrodes are in the electrolyte, so the transfer of electrons (current generated) becomes stable. For this reason, stable concentration measurement is possible even for water to be measured that has "low conductivity" _or "changes in conductivity". * The method of directly measuring the electrodes in the water to be measured has many restrictions on conductivity _and water flow. * Injecting sodium hypochlorite will increase the conductivity of the water to be measured, but it may not always be possible to measure stably.