For data transfer between a host device and communication apparatus via a radiocommunication network and also for data transfer between an external communication network outside the radiocommunication network and the communication apparatus, a relation of connection between the communication apparatus and host device according to a radiocommunication protocol held in the communication apparatus (S1 and S2), communication setting information on the external communication network held in the communication apparatus is used to set a relation of connection between the communication apparatus and external communication network according to an external communication protocol (S3 to S9), and relation of connection between the communication apparatus and host device and that between the communication apparatus and external communication network are used to transfer data between the external communication network and host device (S10 to S12).