We have been developing a task-based service navigation system that offers to the user for his selected services relevant to the task the user wants to perform. We observed that the tasks likely to be performed in a given situation depend on the user's role such as businessman or father. To further our research, we constructed a role-ontology and utilized it to improve the usability of task-based service navigation. We have enhanced a basic task-model by associating tasks with role-concepts defined in the new role-ontology. We can generate a task-list that is precisely tuned to the user's current role. In addition, we can generate a personalized task-list from the task-model based on the user's task selection history. Because services are associated with tasks, our approach makes it much easier to navigate a user to the most appropriate services. In this paper, we describe the construction of our role-ontology and the task-based service navigation system based on the role-ontology.