To effectively inhibit corrosion in a steam boiler and a condensate passage and scale formation in the steam boiler regardless of addition of agents to the boiler feed-water.

The make-up water is treated by a water softening device 53 having a group of resin units including a plurality of resin units filled with sodium cation exchange resin, a cross-flow type filtration treatment device 55 having a nano filtration film, and a deoxidizing device 56, and the deoxidized and decarbonated softened water is stored in a water storage tank 40 as the boiler feed-water. A concentration of hard components in the make-up water, acid consumption (pH 4.8) and a water temperature of the make-up water are respectively measured between the water softening device 53 and the filtration device 55, the resin unit is switched to another one in the group of resin units when the concentration of hard component is over a prescribed concentration, the blow amount from the filtration device 55 is increased when the acid consumption (pH 4.8) is over a prescribed value, and the make-up water is passed through the deoxidizing device 56 while lowering a flow rate of the make-up water when the water temperature is lower than a prescribed temperature.