The Hiroshima debris flow disaster on August 19-20th 2014 exerted serious damage to many houses on the alluvial cone. The authors investigated the debris flow disaster, especially to grasp different formation of the alluvial cone. According to results, the difference in grain size distribution of the debris flow deposit can be attributed to the difference in geological features of the mountainous basins. Consequently, we found out the differences in downstream slope angle of the respective alluvial cone. Thus, the alluvial cone in a granite basin area is relatively gentle slope and the debris flow reaches a far place because the grain size is distributed wide range. On the other hand, the alluvial cone in hornfels basin area is relatively steep slope and the debris flow reaches a narrow place because the grain size is distributed narrow range. Therefore, the appropriate land use on the alluvial cone in a city is important for disaster prevention and mitigation.