A thermal swing extraction system using a thermosensitive gel, poly-N-isopropylacrylamide (NIPA) crosslinked with 2,6-di(3-vinylbenzyl-1,2,4-triazol-5-yl)pyridine (BTP) was proposed. Extraction of Cd(II) in an aqueous nitrate solution and elution with acidic solutions (pH = 1 to 5) were tested by two thermosensitive gels prepared by the ultrasonic polymerization and the ordinary radical polymerization, called US-gel and FR-gel, respectively. Cd(II) was extracted in the gel shrunken at higher than LCST (lower critical solution temperature, 31degreesC) and released from the gel swollen at less than LCST. Such temperature-change of extractability for the US-gel was larger than that for the FR-gel. The US-gel, in which aggregates of BTP molecules are not formed, is suitable to the thermal-swing extraction system. The elution of Cd(II) into a weakly acidic solution (pH > 2) was promoted by the volume phase transition of gel from the shrinking state to the swelling one. Even in a very weakly acidic solution of pH 5, more than 50% of Cd(II) extracted was eluted by the volume phase transition of gel. These results suggest that the elution operation with the volume phase transition of gel is very effective for reducing a quantity of elating agent.