[en] Diffusion property of iodide ion in compacted bentonite containing Ag₂O was investigated as an attempt to improve the safety of the environment when the high level radioactive waste is disposed. Transport of iodide ion was observed by using through-diffusion method in both the compacted bentonite containing Ag₂O and the compacted bentonite without Ag₂O. The compacted bentonite with the density of 1.41 g/cm³ and the one with the density of 1.60 g/cm³ were used in the experiment. The amount of Ag₂O added in compacted bentonite was in the range of 0.0064 ∼ 0.0468 wt/wt%. The solution in which iodide ion was dissolved in demineralized water and the one in which iodide ion was dissolved in 0.1 M NaCl solution were respectively used as diffusion solutions. From the experimental data, by using Fick's equations, time lag against transport, effective diffusion coefficient and apparent diffusion coefficient were obtained and their properties were evaluated. It is confirmed that iodide ion is transported in the compacted bentonite containing Ag₂O and the compacted bentonite without Ag₂O by diffusion process. The migration time for iodide ion to move through the compacted bentonite containing Ag₂O increased with the amount of Ag₂O. The increase of migration time was much more in the solution in which iodide ion was dissolved in demineralized water than in the solution in which iodide ion was dissolved in 0.1 M NaCl solution. However, the relationship between effective diffusion coefficient and the amount of Ag₂O added was not clearly estimated from quantitative viewpoints in this study