The self-sealing function of the clearances between buffer material and overpack and tunnel wall or disposal pit wall and between backfill material and tunnel wall is expected for bentonite which will be used as buffer material and part of the backfill material. In this study, an analytical theory on the clearance filling performance for both materials was constructed. Volumetric swelling ratio of bentonite (Rvs) and maximum clearance ratio that bentonite can fill clearance (Rsmax) were calculated against dry density for different montmorillonite and silica sand contents of bentonites in distilled water and saline water conditions (Horonobe groundwater and synthetic seawater). Both Rvs and Rsmax values decreased with decreasing montmorillonite content and decreased in saline water conditions. The Rvs values in the Horonobe groundwater (ionic strength IS = 0.207 M) were approximately a half of those in distilled water and those in synthetic seawater (IS = 0.64 M) were approximately a half of those in the Horonobe groundwater. In the case that actual sealing performance is judged, not only clearance filling, but also hydraulic conductivity etc. after the clearance was filled must be considered. In the design of buffer material and backfill material, those parameters must be systematically taken into account.