We set the rules for occurrence and non]occurrence of debris flow using the existing site survey data for the sediment disaster in Hofu, Yamaguchi, Japan on July 21, 2009. Using only separable factors for the occurrence and non]occurrence events, we could improve the rule's accuracy rather than in case of using matching rates based on rough set theory. Compared with the accuracy of the rules extracted from topographical or geological maps etc, these two rules' accuracy was equivalent. Therefore, we can detect high]risk torrents of debris flow without interpreting topographical or geological maps. As the results, it is able to reduce the risk of damage at future heavy rainfall by using the extracted rules based on the existing site survey data. Moreover, we can apply this rule to another area with similar conditions. It will be very effective for setting up a plan of disaster]prevention measures.

Key wordsFRough Set Theory, existing site survey data, occurrence and non]occurrence rule, debris flow