The removal efficiency of methyl iodide for silver-impregnated alumina from gaseous waste has been experimentally evaluated as a function of atmospheric relative humidity. A new adsorbent has been developed for the iodine filter installed in the off-gas treatment system of a radioactive waste tank vent. To improve its removal efficiency under a highly humid atmosphere, the optimum average pore size of alumina was determined to be ∼60 nm, and the most effective chemical form of the impregnated silver was identified as silver nitrate. Holding capability of the impregnated silver was also improved by developing a double-pore-structure alumina.