To improve dissolving rate and solubility, to reduce insoluble residue and to prevent the generation of NOx gas in nitric acid dissolution to the utmost at the time of dissolving a multiple oxide of uranium/plutonium containing impurities in nitric acid.

At the time of dissolving the uranium/plutonium multiple oxide in nitric acid, sintered pellets are prepared by adding and mixing 0.2-0.3 wt.% MnO2 after mixing to uniformalize the powder of the uranium/plutonium multiple oxide containing the impurifies or mixing to uniformalize after adding 0.2-0.3 wt.% MnO2, compacting the mixed powder and sintering under a reducing atmosphere (in a hydrogen atmosphere diluted with an inert gas). The sintered pellets are dissolved in nitric acid as it is. Otherwise, the sintered pellets prepared in this way, are oxidized at 300-350°C in the air and dissolved in nitric acid.