Control apparatus, control method, communication system and program

A control apparatus comprises: a determination means that, when a mobile communication terminal, which stores first identification information allocated in a first mobile communication network, uses, in an area where a second mobile communication network can be used, the first mobile communication network via the second mobile communication network, determines whether it is possible to change identification information, which is to be used by the mobile communication terminal to allow the terminal itself to be identified in the second mobile communication network, from the first identification information to second identification information allocated in the second mobile communication network; and a notification means that, when it is determined that the change of the identification information is possible, provides a notification to the effect that the change of the identification information is to be performed to a request apparatus connected to the second mobile communication network and requesting the mobile communication terminal to change the identification information.