PURPOSE: To contrive the relief of a corrosion environment by connecting a recirculating passage in which part of treated off-gas is circulated to an inlet gas passage of a dissolution off-gas device from the downstream portion of an exhauster.

CONSTITUTION: A dissolution off-gas produced from a dissolver 5 of spent fuels is allowed to flow in an NOx treatment part 1, and NOx is absorbed by water herein to recover as nitric acid. Next, iodine in the gas is absorbed and eliminated by an iodine elimination part 2 to exhaust the gas from a stack 4 to the outside by an exhauster 3. Part of the gas is returned to the inlet part of the treatment part 1 through recirculation tubing 7 as the gas allowed to flow out from the exhauster 3 is controlled by a flow control valve 6. A shearing machine for shearing spent fuels is set in the upstream side of the dissolver 5. Air is enforcedly leaked by the shearing machine and sucked and exhausted by the exhauster 3 from the dissolver 5 to maintain negative pressure. Thereby, the corrosion by the residual iodine is prevented and the long life of the device can be contrived without the use of extreme anticorrosion materials.