The present invention is directed to a phosphoric ester of 2,3-dihydroxypropyl cellulose or 3-(2,3-dihydroxypropoxy)-2-hydroxypropyl cellulose, the cellulose having optionally undergone partial carbamidation, and to a metal absorbent employing the ester, and to a metal-absorbing apparatus employing the metal adsorbent.The cellulose derivative phosphoric ester serves as a metal adsorbent which exhibits high metal adsorption performance and metal adsorption rate per unit weight; has high mechanical strength; effectively soften water or a similar liquid; removes heavy metals from wastewater or a similar liquid at remarkably high efficiency; has excellent processability and can be formed into a variety of shapes and thus is applicable within a wide range; and can readily be reutilized.