Running out process of debris in an actual basin is discussed, based on the results of field survey and numerical analysis. In Gamaharazawa river, a tributary of R. Hime, a hazardous debris flow which was triggered by a landslide took place in December 6, 1996. Field survey and numerical analysis are conducted in order to investigate debris flow processes such as occurrence, development, and so on. The numerical results show that the slid debris deposits along the reach and forms a debris flow due to water supply. The flow discharge increases rapidly by eroding the slid debris and original river-bed materi.