As an organic iodine remover that removes organic iodine in a containment vessel of a nuclear reactor, an organic agent (for example, an ionic liquid, an interfacial active agent, a quaternary salt, or a phase transfer catalyst) having a function of dissolving and decomposing the organic iodine and retaining iodine is used. The organic iodine remover is a substance composed of a cation and an anion. The organic iodine remover is, in particular, an organic iodine remover in which, in a structure of the cation of the organic agent, carbon or oxygen is bonded to, via a single bond, to a phosphorus element, a sulfur element or a nitrogen element, the number of carbon chains is 2 or more, and an anionic structure is configured with a substance with high nucleophilicity. By using such an organic agent, the organic iodine is removed with an efficiency of 99% or more.