PURPOSE: To provide lightweight high-porosity adsorbent for effectively removing a minute quantity of phosphate ions, which has low resistance to the passage of water and is unlikely to close its porous with suspended substance.

CONSTITUTION: In a phosphate ion adsorbent 1 in which a phosphorus-adsorbing substance is fixed by polymer gel 2 in a three dimensional network, the gel is preferably a natural polymer gel such as alginic acid and chitosan. The three dimensional network is permeated by an aqueous solution of organic polymer containing fine particles of a phosphorus-adsorbing substance, and the solution is converted into gel to make the adsorbent.