[en] Four kinds of diffusion experiments; (1) through-diffusion (T-D) experiments for compaction direction dependency, (2) in-diffusion (I-D) experiments for composition dependency of silica sand in bentonite, (3) I-D experiments for initial bentonite grain size dependency, and (4) I-D experiments for the restoration property of an artificial single fracture in compacted bentonite, were carried out using tritiated water which is a non-sorbing nuclide to evaluate the effect of pore structural factors for compacted bentonite on diffusion. For (1), effective diffusivities (De) in Na-bentonites, Kunigel-V1 and Kunipia-F were measured for 1.0 and 1.5 Mg·m^-3. For (2), apparent diffusivities (Da) in Kunigel-V1 were measured for 0.8, 1.4 and 1.8 Mg·m^-3 with silica sand of 30 and 50 wt%. For (3), Da values for 0.8, 1.4 and 1.8 Mg·m^-3 were measured for a granulated Na-bentonite, OT-9607 which grain-size distribution is in a range between 0.1 and 5 mm. For (4), Da values in Kunigel-V1 which a single fracture was artificially reproduced and was immersed in distilled water for 7 or 28 days for the restoration of the fracture, were measured for 1.8 Mg·m^-3. Although De values in Kunigel-V1 were approximately the same for both compacted directions over the density, De values for perpendicular direction to compacted direction were higher than those for the same direction as compacted direction in Kunipia-F. For composition dependency of silica sand in bentonite, no significant effect of the mixture of silica sand in bentonite on Da was found. For initial bentonite grain size dependency, Da values obtained for OT-9607 were approximately the same as those for Kunigel-V1 and no effect of initial grain size of bentonite on diffusion was found. For the restoration property of a single fracture in compacted bentonite, no restoration period dependency on Da was found. Based on this, it may be said that diffusion of nuclides in compacted bentonite, Kunigel-V1 is independent of compaction direction of bentonite, composition of silica sand in bentonite, and initial bentonite grain size. Furthermore, no matter how a penetrated fracture was formed in bentonite, the fracture is restored for a short while. (author)