This paper proposes a service model based on the service hierarchical structure called Galaxy Service Model, and a service discovery framework called Galaxy Service Directory System. In a ubiquitous computing environment, software services are embedded into various devices. The application construction requires service discovery mechanism and a recursive definition of services. The service discovery mechanism should, therefore, provide applications with common interface to look up the hierarchically represented composite services. In existing service framework, applications cannot be combined into other applications, since they do not provide the recursive service representation facility. In contrast, a service in Galaxy has a hierarchical description by deploying nested services. This model makes other services and applications to deal with an application that is composed by services. Galaxy service directory system facilitates structure-bounded service registration and service discovery. The system enables applications to find different composition-levels of services through a common interface.