objects to be decontaminated further oxidizing agent need not be decomposed during the period of decontamination so metal ion generated by decomposition of oxidizing agent need not be absorbed and removed by the cation resin with the result that cation resin adsorption is decreased for example 200 ppm of potassium permanganate is used as an oxidizing decontamination agent and 10 potassium permanganate is replenished in each cycle upon decomposition of four objects the oxidizing agent is decomposed and the manganese ion and potassium ion resulting from decomposition are absorbed and removed by cation resin if the surface area of one object to be decontaminated is 40 m 2 and the amount of oxidizing decontamination agent is 3 m 3 then the amount of adsorption of potassium ion and manganese ion generated by decomposition of oxidizing agent in the cation resin can be reduced to about 11 of the total adsorption amount of cation resin this is a substantial reduction in the adsorption of resin as compared to the percentage of the prior art it should be noted that the effect in reducing the amount of chemicals is increased with the number of objects to be decontaminated in the present embodiment the radioactive concentration of the reducing decontamination tank 2 a is controlled at a higher value and that of reducing.