An automatic analyzer detects voltage applied across electrodes, and judges whether voltage value falls within set voltage range. When the detected voltage value is lower than minimum value of set voltage range, the analyzer calculates the deficient amount of base solution based on the detected voltage value, controls a valve to supply the deficient amount of base solution, then, performs operation control of the valve so as to keep the prescribed amount of plating solution in plating solution tank, and discharges plating solution. When the detected voltage value is higher than maximum value of set voltage range, the analyzer calculates the excess amount of base solution based on the detected voltage value, controls a valve, and supplies pure water into the tank so that the base solution concentration falls within prescribed range to dilute plating solution, then controls a valve, and discharges plating solution so as to keep prescribed amount.