To provide a water treatment agent for a boiler device which can prevent the generation of corrosion without causing the coloring of boiler water and the increase of COD value, and to provide a water treatment method for a boiler device.

A water treatment agent comprising: an oxygen scavenger composed of at least one of organic compound selected from the group consisting of erythorbic acid, erythorbate, ascorbic acid and ascorbate; and neutral amine such as alkylamine and alkanolamine is continuously added while feeding water into a boiler device.