The emergence of the Internet has broken down geographic and organizational boundaries, providing a virtual common workplace regardless of the heterogeneity of participating organizations. Enterprise projects that used to be done autonomously now span multiple organizations. While an inter-organizational workflow, as one of several technologies supporting inter-organizational collaboration, provides an easy-to-use collaborative work environment for users, it also increases the complexity of security maintenance and brings about security problems that were not considered before. Unconventional collaborations among businesses and organizations are formed to advance common goals. In this paper, we address the security services to support inter-organizational collaborative enterprises, which may span multiple organizations, and describe how we develop a secure workflow system to satisfy the requirements by integrating with existing, wellknown technologies. Although we apply our ideas to particular technologies, such as workflows and RBAC, in this paper, we believe it is always possible to apply our approaches to other systems, which support many users from different organizations.