Weakly acidic hypochlorite is used for multiple purposes in the factory. Monitor the "residual chlorine" and "pH" of the sterilizing water that intermittently flows from the weakly acidic hypochlorous water generator. Since the amount of flowing water increases or decreases depending on the operating conditions of the factory, the conventional "residual salt monitoring sensor" has a problem that the indicated value changes significantly and is not stable. In addition, when the water stops, the water flowing to the residual salt sensor also stops, so that the indicated value drops sharply and the alarm of "residual salt abnormality" is often issued, which is a problem. In the generator, since carbon dioxide gas is injected, there is a problem that the sensor measurement unit tends to foam and the indicated value is not stable.