To provide a protocol change-over system which connects to a destination side by selecting a communication protocol corresponding to the destination side who called out an own system, without being affected by the model of a modem to be connected.

A call termination message table 13 memorizes a plurality of call termination messages so that the modem may notify that call-out is made from a center, while a communications protocol table 15 transfers to a telephone number of the center and memorizes the communication protocol. A controlling unit 17 outputs a response command to connect to the destination side to the modem only when the call termination messages notified from the modem through an interface 19 are memorized on the call termination table 13, searches the communication protocol table 15 by using the telephone number included in the call termination messages as a search key, and then determines the communication protocol corresponding to the telephone number.

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