Bentonite-based buffer materials for high-level nuclear waste disposal are expected to fill up the space between buffer and a wall of the disposal pit, and/or between buffer and an overpack by its swelling deformation. That is called as self-sealing ability. This study performs the one-dimensional model tests simulated the relationship between buffer and space mentioned above. It also investigates the validity of the new equations for evaluating the swelling characteristics of bentonite-based buffer material, which were proposed the first and second authors, by comparing the calculations and the experimental results. The experimental works and calculations showed the filling-up of spaces by swelling of buffer, and the high applicability of authors' equations to evaluate self-sealing ability.