As the neutral amine employed in the present invention, cyclohexylamine, 2-amino-2-methyl-1-propanol, monoethanolamine, diethanolamine, morpholine, monoisopropanolamine, diethylethanolamine, diethylpropanolamine, dimethylethanolamine, and dimethylpropylamine may be preferably employed, but the neutral amine is not limited to these. Each of these may be suitably used alone or in combination with one or more of the others.

Though the oxygen scavenger of the present invention can be prepared by mixing the aforementioned heterocyclic compound having the N-substituted amino group or the salt thereof, one or more of the hydroxybenzene derivatives and/or the naphthoquinone derivatives (hereinafter, sometimes called as "catalytic compound"), and the neutral amine, these may be separately added to feed water.