To provide an effective corrosionproof means not bringing about a new corrosion problem by an economical and inexpensive method.

The inner surface corrosion caused by a water quality problem in the inner wall of the heat transfer pipe constituting a horizontally arranged evaporator or overheat device can be prevented using the following supply water, that is, boiler supply water (1) obtained by pretreating supply water in a purity enhancing device 15 of boiler supply water comprising a deionizing device utilizing a pretreatment device 14 having a sedimentation tank, a filter tank, an adsorbing tank or the like to heat the same to 150°C or higher and subsequently cooling treated water to deionize the same, boiler supply water (2) obtained by pretreating supply water to deionize the same and heating the deionized water to 150°C or higher before cooling the same, boiler supply water (3) obtained by arranging a heating and oxidizing device 17 in the fire oven in a boiler or the exhaust gas duct of a waste heat recovery boiler or boiler supply water (4) using a part of steam generated from the boiler as the heating source of the heating and oxidizing device 17.