In order to build high quality RBAC system, researchers have proposed two important approaches: the top-down approach and the bottom-up approach. The top-down approach [3,4] often starts with expert analysis of business processes and builds RBAC system from such analysis. However, the top-down approach is time consuming since it is human-intensive [5]. The bottom-up approach can discover roles from existing user-permission assignments automatically. Such a computing-intensive approach is called role mining.