PURPOSE: To enable a regeneration of ion exchanging rein for a water softening device for a boiler to be automatically carried out and provide a substantial reduction of expenditure required for the regeneration of ion exchanging resin.

CONSTITUTION: In the case that a water softening operation for supplied water for a boiler through a thermal regenerating type ion exchanging resin Z through one water softening device of a first water softening device 3A and a second water softening device 3B is being carried out, a regenerating action with heat held in discharged gas of a boiler is performed by the other water softening device and then water softening action and regenerating action are alternatively carried out by each of the water softening devices 3A and 3B.