To provide a deoxidation method by which deoxidized water can easily and securely be obtained, and to provide a deoxidation device.

In the deoxidation method, deoxidized water obtained by adding hypophosphorous acid or the salt thereof and formic acid or the salt thereof to the water to be treated and thereafter passing the water to be treated into an oxidation catalyst packed bed is utilized as boiler feed water in a boiler.