PURPOSE: To reduce the number of refining cycles for U and Pu of irradiated fuel within the reprocessing facility by extracting U (VI) and Pu (IV) ions in an acid solution by using a mixture of N, N-dialkylamide.

CONSTITUTION: A solution (in general, nitrate solution) is brought into contact with an organic solvent including first and second N,N-dialkylamide shown by formulae I and II (R1 is a straight-chain alkyl group of 1-12C or H, same or different R2 and R4 are a straight-chain or a branched chain alkyl group of 1-4C; same or different R3 and R5 are a straight-chain or a branched chain alkyl group of 1-6C, a and b are an integer of 1-6; R6 is a straight chain alkyl group of 3-12C), so that the U (VI) and Pu (IV) are extracted into the organic solvent at the same time. The obtained organic solvent is brought into contact with a nitric solution with nitric acid concentration of 0.1-2mol/l so as to extract Pu (IV) in the organic solvent into the nitric acid solution.