PURPOSE: To utilize the heat source efficiently by changing the blow water discharged from an deaerator into thickened water through the treatments of pressure reduction, evaporation, and heat exchanging and supplying to the deaerator the supply water which is a mixture of the condensed water and soft water.

CONSTITUTION: To a blow water discharge device a steam generating and pressure reduction chamber 21 is added, and this steam generating and pressure reduction chamber 21 and a supply water tank 3 are connected by providing a steam pipe 22, condenser 23, and condensed water tube 24 and, at the same time, a supply water pipe 10 of the supply water tank 3 is made to penetrate the condenser 23 and it is connected to a deaerator 4. And, the pressure of the blow water that is discharged from the deaerator 4 is reduced in the pressure reduction chamber 21, and part of the blow water is evaporated in the evaporation pipe 22, and that steam exchanges heat with the supply water indirectly in the condenser 23 to become condensed water. The supply water which is the mixture of this condensed water and soft water is supplied by circulation to the deaerator 4 through the supply water tank 3. With this arrangement there is no increase in the supply water, and it is possible to utilize the heat source effectively.