To restore normal data communication without disconnecting a communication line even in the case of a quick variance of a reception signal.

After the end of negotiation, a coefficient control part 7 sets the variable mode, where parameters of an AGC 2 and an equalizer and echo canceller 3 are variably controlled, for a certain time. Thereafter, it sets the fixed mode where parameters are fixed to values just before the variable mode (or average values in the variable mode). A demodulation part 6 demodulates a reception signal DRS and outputs it as a demodulation signal S1 to a circuit in the succeeding stage. Consequently, the device is not affected by disturbance even in the case of quick disturbance to the reception signal DRS because parameters set to the AGC 2 and the equalizer and echo canceller 3 are fixed to values just after the completion of negotiation, and disconnection of the communication line is avoided.