A lot of slope failures and debris flows occurred at Shobara on July 16, 2010. Some characteristic phenomena were seen by the course that the failure materials flowed down. In many places, surface soils were left without being eroded deeply, though debris materials passed on them. To clarify the cause, we have made some experiments with the soils of Shobara. From the analysis using the X-ray diffraction device, we found out that vermiculite with the swelling characteristic was mainly included. From the swelling check examination between 1000 hours, water more than the volume of initial void before the test was absorbed. Also, from the measuring of the swelling pressure, the maximum value of 9.68 kPa was recorded between 500 hours. These results suggest that the soils of Shobara remained in a kind of unsaturated condition in spite of heavy rainfall during 3 hours, therefore, they were not eroded deeply.