To provide a method for enabling to judge the ion selectivity of an ion exchanger easily.

The ion selectivity of the ion exchanger is judged based on the amount of a non-freezing water existing in the inner portion of the ion exchanger. In more concrete, a plurality of ionic species are absorbed respectively to the ion exchanger and the amount of non-freezing water existing in the inner portion of the ion exchanger, to which each ion is absorbed, is measured respectively. As the non-freezing water amount in the ion exchanger, which absorbs a certain ion, is more, the ion selectivity of the ion exchanger against its ion is judged to be weak.