Provided is a deoxidizing agent for boilers, which exhibits excellent oxygen removal effect under wide temperature conditions. A deoxidizing agent for boilers, which contains (A) a hydroxylamine compound represented by general formula (I), (B) a heterocyclic compound having an N-substituted amino group, and (C) an aminophenol derivative represented by general formula (II). (In formula (I), each of R1 and R2 independently represents an alkyl group having 1-5 carbon atoms.) (In formula (II), each of R3-R6 independently represents (a) a moiety represented by general formula (III), (b) -OR10 or (c) -R11, and at least one of the R3-R6 moieties represents (a); and each of R10 and R11 independently represents a hydrogen atom or an alkyl group having 1-4 carbon atoms.) (In formula (III), R7 represents a single bond or an alkylene group having 1-4 carbon atoms; and each of R8 and R9 independently represents a hydrogen atom or an alkyl group having 1-4 carbon atoms.)
