PURPOSE: To provide a method whereby the concentration of ions of a specific component included in a sample liquid can be correctly measured in a short time without consuming a large quantity of cleaning liquid.

CONSTITUTION: Before a sample liquid 1 is supplied to a measuring cell 11 to measure the concentration of ions of a specific component included in the sample liquid 1, a high concentration liquid 8 which shows a higher ionic concentration than a standard liquid 5 is fed to the measuring cell 11, and then, the standard liquid 5 is sent to the measuring cell 11 consecutively twice. The amount of contamination of the high concentration liquid 8 is obtained from potentials of the standard liquid 5 sent at the first and second times, based on which the ionic concentration obtained at an operating part 18 is corrected.