To provide a method capable of dissolving more radioactive cesium into water, strongly holding it therein and enhancing decontamination efficiency.

The method comprises: contacting water with a vessel charged with dielectrics 2 (ceramic particles, etc.); maintaining an electrical phenomenon generated by an interfacial electrokinetic phenomenon or a thermoelectric conversion effect (pyroelectricity and piezoelectricity, etc.) caused by allowing dielectrics 2 to flow, rub together and collide by a passing flow, etc., of a liquid, as positive charges and hydroxide ions in the water to structure the water; dissolving metal ions such as cesium more in the water rather than in normal water by increasing an amount and dissolving power of carbon dioxide dissolved in the water and hydroxide ions by structuring the water to enhance dissolving power; and increasing a function of the water as a transport medium of the metal ions such as cesium in order to stably fix, especially, radioactive cesium to FESs (flayed edge site) of a soil mineral, etc., to enhance decontamination efficiency.