To prevent corrosion by injecting oxygen to a low pressure and high pressure feed water heater drain line to increase the oxygen concentration in equipments and pipe lines.

In a plant applying an oxygen treating method as the corrosion countermeasure, an oxygen injection pipe line 26 for injecting oxygen to the low pressure feed water heater 8 and the high pressure feed water heater 13, which are installed in a water feed line, is provided and oxygen is injected to the drain line of each feed water heater. Alternatively, steam from a bleeding pipe line 19 is supplied to the inside of the low pressure feed water heater 8 and the high pressure feed water heater 13, which are installed in the water feed line, and the steam is heat-exchanged with a feed water in a water feed pipe line arranged in each feed water heater, the drain of steam is introduced into drain lines 16, 17 and oxygen is injected to a part of a liquid phase in the inside of each feed water heater separated into a gas phase and the liquid phase.