A method of measuring the concentration of a solution is described. In this method, electrodes are installed on the outside of the pipe or vessel and the concentration is measured indirectly through the vessel material by detecting the capacitance change between the electrodes which is caused by the variation of the liquid conductivity. The measuring circuit is stable as it has a temperature compensating circuit, which, in addition, serves to control the concentration without regard to the temperature change, using another pair of electrodes which are filled with the standard solution. Concentration changes of a solution, with electric conductivity of 0.5∼10 k&OHgr; cm, can be measured with a sensitivity of 5 &mgr;A/1%. The output caused by a temperature change of 10 C° and the stability in a day are both less than 1 &mgr;A.