To realize communication between different mobile communication systems by down-loading software using software radio technology containing software defining a radio function from a reception line used for radio communication, storing it, starting the software and executing a radio base band processing.

A network requests the version of terminal software and a terminal returns the version. The network sends a down loading start command and the terminal returns the response. Whole data is divided and they are down- loaded on the terminal from the network. Errors are properly checked during down loading. When the down loading of whole data is terminated, the network sends a down loading termination command and the terminal responds so it. The network sends a software switch command and the terminal switches software which is down-loaded.