PURPOSE: To securely discriminate a special control command without confusing with other data by discriminating whether it is the special command or not from the transfer speed of data.

CONSTITUTION: A control part 11 regards signals on an SD line as commands in the state of accepting the commands. In such a time an ST line becomes a level, L and the signal on an SDC line is outputted to a CPU through a RD line. Then, in the state of transmitting and receiving data, the ST line becomes a level H, and data inputted from the SD line is once inputted to the control part 11 and the transfer speed of data is checked. Next, data is outputted to the SDC line, is inputted in a modulation part 19 to be modulated, and outputted to a telephone line through a telephone line interface 18. Thus, the special command is securely discriminated without being confused with other data.