PURPOSE: To sample U especially from the sea water, by contacting the U dissolved solution with chelate ion exchange resin having metal ions absorbed to absorb dissolved U, and then contacting with NH4Cl, (NH4)2CO3, or the like.

CONSTITUTION: Chelate ion exchange resin having metal ions such as Ti and iron adsorbed is contacted with U dissolved solution of low concetration such as sea water and waste water from nuclear reactor, and dissolved U is adsorbed. In this adsorption process, it is preferable to keep the value of pH of U dissolved solution at 4 to 8, especially under weak acidic state, and the adsorption capacity is further increased when the water temperature is kept above 50°C. Then, the chelate resin with metal ions having U adsorbed is contacted with aqueous solution of more than one kind of salts selected from NH4Cl, Na2CO3 and (NH4)2CO3 to desorb U. The concentration of salt at this time should be preferably 0.5 to 2 mols, and it is advantageous to perform desorption operation at higher than pH 4. As for metal ions, it is preferable to use Ti ions. As for the chelate resin, chelate ion exchange resin of which chelate forming functional group is polyamine is used.