PURPOSE: To sufficiently perform the operation of equalization for automatic equalizer at reception side, by transmitting the synchronizing signal over plural numbers according to the characteristics of transmission line, in transmitting the synchronizing signal.

CONSTITUTION: The same synchronizing signals 1A∼1C are transmitted three times according to the characteristics of transmission line and the data signal 2 in 9,600 bit/sec is transmitted. Thus, the automatic equalizer at the reception side is sufficiently operated for equalization, then the data signal 2 in 9,600 bit/sec is surely received and the response signal 5 is immediately transmitted, succeeded with the synchronizing signals 1A∼1C. Accordingly, the MODEM at the transmission side is set to the propagation speed in 9,600 bit/sec and the transmission of the data signal 2 in this speed is continued.