A manufacturing method of water-absorbent agent powders of the present invention is a method of a reducing an amount of a residue of an epoxy crosslinking agent remaining therein by adding a nucleophilic reagent in a form of powder to surface region crosslinked water-absorbent resin powders having a carboxyl group under an applied heat in which the residue of the crosslinking agent remains. Since the method permits an amount of the residue of the crosslinking agent to be reduced by adding a nucleophilic reagent to the heated water-absorbent resin powders, the water-absorbent agent powders exhibiting well-balanced properties which are mutually negatively correlated from one another, i.e., high absorbency under pressure, a reduced amount of a residue of the epoxy crosslinking agent and a high absorbing rate compared with the conventional surface region crosslinked water-absorbent resin powders can be achieved. Such water-absorbent agent powders are suitably used in sanitary materials such as disposable diaper, sanitary napkins, etc.