Ubiquitous technology - mainly represented by RFID (Radio Frequency Identification) and USN (Ubiquitous Sensor Network) technology - has been attracting public attention in various fields of industry in the twenty-first century. In the meantime, people have begun to think of real and ultimate ubiquitous services providing the convergent services interconnecting several systems in different service areas. In order to connect various resources and services in the u-environment, the ID system is important in that resources including even creatures and plants will be given IDs and managed by the ID system for the incorporation of u-services. However, it is a very difficult problem to interoperate two different services because they use different ID systems. This paper introduces the basic concept and various types of ID systems with some issues for interoperation among ID systems. The main part of this paper proposes a method for enabling interoperation between two different types of ID systems such as digital resource and physical resource ID systems in terms of 'identifiers' and 'ID management system'. Finally, a conceptually exemplified case is provided for later application. New services generated by the method proposed in this paper will be a potent catalyst to expand ubiquitous technology, considering that tangible and profitable u-services are still limited.