Bentonite-based buffer materials are expected to reduce stress from rock masses and mitigate nuclide migration in the geological disposal of high-level radioactive waste of Japan. Such performance is achieved by ensuring buffer density and thickness based on the swellability characteristics of clay. However, we should also consider bentonite buffer piping erosion as a phenomenon in which the buffer surface is destroyed due to the effects of groundwater flowing between the buffer and the rock. That is, the buffer's clay content is removed by the groundwater flow with its suspension, thereby reducing its mass. Such piping erosion is a serious issue for maintaining an engineered barrier in the geological system for radioactive wastes. This study examined the dynamic behaviors of the piping erosion experimentally. In the experiments, 500 mmφ × 600 mm height bentonite buffers were placed into a cylindrical acrylic cell with 560 mmφ in inner diameter and 600 mm in height. Distilled water was continuously injected at a flow rate of 0.1 L/min, discharged from the upper surface, with the bottom surface made porous so that it could be injected from the entire bottom surface. In the experiments, the turbidity of the suspended clay in the drainage water was measured to determine the amount of bentonite that flowed out of the cell. In the results, while the bentonite sample immediately swelled and attached to the inside wall of the cell, a dominant flow-channel (piping) was observed between the swelled bentonite sample and the inside wall of the cell. Also, the relationship between the accumulated amounts of injected water and erosion bentonite showed a constant slope on double logarithmic plots. These mean that the shear stress due to the water flow locally exceeded the shear strength of swelled bentonite and the piping erosion proceeded with a simple regularity. Furthermore, this study tried injecting distilled water through a local inlet and outlet set on the side of the cell, in order to earlier form a flow-channel between the bentonite sample and the inside wall. The results also showed almost the same slope in the double logarithmic plots of the accumulated amounts of injected water and erosion bentonite. Additionally, the slopes already reported with a smaller size cell were quite similar_ to those in this study. The results suggest that the piping erosion in actual-scale bentonite buffers (e.g., 2.22mφ×4.15 m) can be predicted from small-scale data.