This invention is to enable to provide a network service, flexibly and quickly. In an embodiment of the invention, a network platform is achieved by a service providing network. The network platform provides common functions such as authentication and authorization, billing, and client management, and individual functions such as session control for client terminals connected with the service providing network. The client terminal generates and registers a service program to utilize functions in the network platform via a network management interface SMI. Then, by executing the service program, a service request for a specific function is transmitted to the network platform through a service control interface SCI, and the execution result is received from the specific function in the network platform.