To accurately detect whether a specified brine is sent to an ion- exchange resin bed or not when a softener is regenerated without frequently calibrating a conductometer.

This softener is provided respectively with a resin tower 7 packed with an ion-exchange resin in a vessel, a brine tank 9 for generating a brine, a control valve 8 for controlling water supply and a controller 10 for controlling the control valve 8 to collect soft water. The ion-exchange resin bed in the resin tower is periodically applied to a regenerating process consisting of a backwashing process, a chemical dosing and extruding process and a cleaning process and the water used in the regeneration is discharged through a water discharge line 12. In this case, an conductometer 4 is provided to the water discharge line 12 to measure the conductivity of water and to output the measured value to the controller 10. The conductivity of the waste water is measured in each of the processes at the backwashing stage and the chemical dosing and extruding stage to calculate the difference of electrical conductivity between the both processes and the abnormality in the regeneration is outputted when the calculated difference is smaller than the preset value.