PURPOSE: To obtain an adsorbent which adsorbs uranium selectively from sea water, has a high rate of adsorption and has excellent durability by forming the uranium adsorbent of a polymer of a porous and huge network structure having a phosphonium and a phosphine group.

CONSTITUTION: A polymer of a porous and huge network structure having an arom. ring is allowed to react with phosphorus trichloride and is then reacted with chlorine, whereafter the polymer is hydrolyzed, whereby an uranium adsorbent of the porous and huge network structure having a phosphonium group and a phosphine group is produced. The same uranium adsorbent as mentioned above is also obtainable by subjecting the polymer of the porous huge network structure having the arom. ring to chloromethylation by an ordinary method then allowing the same to react with phosphorus trichloride and hydrolyzing the same. When the uranium adsorbent is brought into contact with an uranium-contg. soln., the phosphonium group forms stable chelate with uranium and the uranium is adsorbed with high selectivity.