[en] Purpose: To enable stable measurement of accurate dissolved oxygen density for a long time in reactor water. Constitution: This device employs, for example, sodium carbonate (stable under high temperature and high pressure state) as the electrolyte, and platinum as the anode and the cathode. In this case, since the reactions at the cathode and the anode are offset with each other, oxygen and acid in the electrolytes are not consumed at all. Further, since no water is formed as well, substantially the same result occurs as a state where no reaction has been taken place. Accordingly, dissolved oxygen density in the nuclear reactor coolants can be measured for a long period of time (more than one year) and at a high stability. (Kawakami, Y.)