Oxygen scavenger which can exhibit excellent oxygen reduction effect in a wide variety of conditions is provided as an alternative volatile oxygen scavenger to hydrazine. The oxygen scavenger contains a heterocyclic compound having N-substituted amino group such as 1-amino-4-methylpiperazine and a hydroxylamine compound such as N,N-diethylhydroxylamine. The oxygen scavenger may further contain a polyhydric phenol type catalyst such as pyrogallol. By adding the oxygen scavenger to feed water of a water system of a boiler, oxygen reduction treatment of the water system is carried out.