PURPOSE: To enable high-quality data transmission by calculating error between a received signal vector and an original vector when training a MODEM, deciding transmission quality and shifting-down data transmission speed when the transmission does not satisfy the constant quality.

CONSTITUTION: After receiving a DCS signal, the training operation of a high- speed MODEM 41 is executed. An error detection circuit 412h outputs an error vector Er showing deviation between the signal vector of the received signal and the original signal vector. An equalizer control circuit 412i controls an equalizer 412d so that the error vector Er can be made minimum. A multiplier circuit 412l outputs the degree of margin to noise on a transmission line at that time, namely, outputs the evaluation value of the transmission quality. When evaluation value data Ed is less than a certain fixed value, the data transmission speed of the high-speed MODEM 41 is shifted down and turned to a signal reception state at the data speed lower for one stage.