A method of oxidatively decomposing a radioactive ion-exchange resin is described, the method comprising oxidatively decomposing a radioactive ion-exchange resin containing an anion-exchange resin with hydrogen peroxide used as an oxidizing agent in the presence of iron and copper ions used as catalysts, wherein the weight ratio of hydrogen peroxide to the ion-exchange resin, that is the ratio of the net weight of hydrogen peroxide to the dry weight of the ion-exchange resin containing an anion-exchange resin, is held to be no higher than 17 and the pH of the reaction system is adjusted to be within the range of 0.5 to 6, or citric acid ions are preliminarily adsorbed on the radioactive ion-exchange resin before it is subjected to decomposition treatment or citric acid ions coexist with the radioactive ion-exchange resin in the oxidatively decomposing system, and an apparatus used for conducting the method is also described.