A portable telephone apparatus that achieves international roaming inexpensively by performing roaming between mobile telephone systems that use different communication protocols. In detail, if the present apparatus, in an initialization state, receives a sync channel message generated from a base station, the portable telephone apparatus reads a P_REV message which shows the communication protocol used in the mobile phone system. When the communication protocol is T53, the portable telephone apparatus performs a plurality of procedures from an idle state through to a traffic channel state in a standard manner, but when the communication protocol is IS-95, the potable telephone apparatus controls wireless communication by replacing one procedure of the plurality of procedures performed from the idle state through to the traffic channel state with another procedure.