To monitor impurities in a boiler water system while simplifying maintenance and reducing managing cost by suppressing the deterioration of an ion-exchange resin, actualizing stable impurity monitoring on a longer-period basis, and reducing the exchanging frequency of the ion-exchange resin in a monitoring period.

The flow of sample water 6 into the ion-exchange resin 4 is controlled to suppress the deterioration of the ion-exchange resin. When the concentration of impurities is not measured, the sample water after ion-exchange is circulated into the ion-exchange resin to suppress a variation of a measured value for the concentration of impurities in the sample water.