PURPOSE: To effectively utilize NOX in a reprocessing by using NOX on nitrous acid generated in the dissolution reaction for spent fuel or radiolysis reaction of nitric acid for controlling the valence of the dissolved material in nitric acid solution in the reprocessing.

CONSTITUTION: In the NOX generation process 1 in a reprocessing nitrous acid is produced due to the dissolution reaction of nitric acid 2. It eventually becomes to NOX and moves to gas phase gas 6. It is transported as it is or after being purified by eliminating the co-existing matter included in it with a co-existing matter eliminating device 5 or being purified to some extent, to a valence control process 3 in the reprocessing process as NOX gas 7. In the process 3, by bowing in NOX, the valence state of solved material in the nitric acid 4 is converted into the state proportional to the electric potential of NOX. Thus, by transporting the NOX generated in the process 1 to process 3, the dissolved material in the process 3 is oxidized or reduced.