PURPOSE: To increase the transmission efficiency of the network by sending back the diverged information of the automatic equalizer in the form of the status information via the secondary channel which uses the single frequency outside the frequency distribution region used for the data transmission.

CONSTITUTION: In case the automatic equalizer of slave station MODEM106 has divergence, the divergence signal of the automatic equalizer is supplied to transmission part 106a of the secondary channel provided to MODEM106. Receiving the diverged information, part 106a transmits the diverged information of the automatic equalizer of MODEM of its own to central station MODEM102 via single frequency 1002 selected outside the frequency distribution region used for the data transmission. When reception part 102a of the secondary channel incorporated into MODEM102 receives the single frequency, the synchronized signal generator circuit of the automatic equalizer incorporated into MODEM102 is actuated. And with this synchronized signal, the re-equalization is given to the automatic equalizer of MODEM106.