In this studies, I describe about a fundamental general idea of rough set that attracted attention as foundations technology to treat the knowledge that can\u27t be distinguished in observation data, and possibility of application of rough set in acquisition of experienced knowledge in observation data. And, I extract very small decision algorithm in observation data of collapse danger degree diagnosis of nature slope, and suggest a method to guide a rules using the result. At the same time, observation data include the case that contradiction is contained, and it is necessary to expel these observation data which contradicted when take down judgment. So, I show it using real observation data about a method to extract these contradiction
