The method according to the present invention comprises using a high-frequency vibrating stirrer that is confirmed to include a treatment tank I, a high-frequency vibrating motor 3 fixed to a table positioned above the treatment tank 1, two vibrating rods 4 extending toward the bottom of the treatment tank 1 and coupled to the table, and multistage vibrating blades 5 mounted to the lower parts of the vibrating rods 4 and surface-plated with palladium or platinum serving as a catalyst in element transmutation, characterized in that the high frequency vibrating motor 3 is controlled by an inverter 6 so as to vibrate the multistage vibrating blades 5 at a frequency of 100-170 Hz in an aqueous solution 2 containing an element to be transmuted in the treatment tank 1, thereby transmuting the element in the aqueous solution 2 into another element. By adding heavy water to the solution to be treated, the transmutation efficiency can be elevated. By adding tritium water with an appropriate concentration as a substitute for the heavy water, the element transmutation can be completed within a short period of time and, at the same time, the tritium water that is seemingly the main cause of radioactive contamination can be effectively utilized and the radioactivity thereof can be attenuated or detoxified.