On March 5, 2020, a slope failure was observed below the wing of the Maeirisawa No. 2 check dam in Shibukawa City, Gunma prefecture. To estimate the process of this failure, we conducted geological surveys and laboratory permeability tests on pumice and sand‐gravel samples collected from the failed slopes. We also installed a time‐lapse camera to monitor the slopes excavated for repair works. The investigation results revealed that a highly permeable pumice layer was interbedded within sand‐gravel layers. Time‐lapse images captured erosion in the exposed sections of the pumice layer during rainfall events, indicating that the layer was more prone to erosion than the sand‐gravel layers. These findings suggest that the primary causes of the failure were the repeated erosion of the pumice layer and the subsequent collapse of the overlying and underlying sand and gravel layers due to rainfall.