Abstract: A water treating agent is constituted by blending water as a main component with a coating film forming agent of which a coating film is formed on the heat transfer surface of a boiler tube, a oxygen scavenger, a scale inhibitor, and a pH adjustor in such a manner that the general corrosion and local corrosion of the heat transfer surface of the boiler tube can be prevented and excellent heat transfer property can be obtained. The water treating agent is injected into boiler feed water to perform the prevention of the corrosion of the heat transfer surface of the boiler tube and the suppression of the scaling of the heat transfer surface in a balanced manner. Thus, a coating film capable of suppressing corrosion is formed of the coating film forming agent on the heat transfer surface of the boiler tube. Dissolved oxygen in boiler feed water is removed by the oxygen scavenger.