PURPOSE: To prevent the corrosion of all systems of a boiler unit by controlling the vent valve of a deaerator by a vent valve controller connected to an oxygen concentration meter.

CONSTITUTION: High-concentration oxygen, separated from feed water at the inlet port of a deaerator 7 under high load of a boiler 12 and stagnated in the deaerator 7, is dissolved into feed water upon lowering of the boiler 12 load. When the concentration of oxygen in the feed water has arrived at an upper limit valve set in an oxygen concentration meter 15, the vent valve 16 of the deaerator is opened automatically by a signal from a vent valve controller 17 and the high concentration oxygen, stagnated in the deaerator, is discharged to the outside of the system. Accordingly, the concentration of oxygen dissolved in feed water can be prevented from becomming excessive whereby corrosion of devices in the downstream of the deaerator 7 can be prevented from being increased. According to this method, the concentration of oxygen dissolved in the feed water can be retained at an optimum value.