Recently the information and network technologies have been applied to the urban environment. We call this type of a city a ubiquitous-city, or shortly a U-City. Security is an important factor to the success of U-City, by protecting citizen’s private date and by preventing malicious attack to the urban facilities. This paper proposes a U-City Integrated Authentication System (UCIAS), which is a unified authentication model installed in a U-City using the ubiquitous role-based access control protocol—the modified version of the original RBAC protocol. The strong point of UCIAS is fast response time, enhanced scalability and reduced workload, which come from the fact that UCIAS rapidly re-authenticates user inter-domain movements without generating a whole dataset again. We built a simple analytic tool to evaluate UCIAS and the results support our assertion.