PURPOSE: To attain data transmission in a transmission speed suitable for the state of a line by providing a means or the like obtaining the S/N and comparing it with a reference value in response to the signal transmission speed to be transmitted.

CONSTITUTION: A demodulator 206 demodulates a reception signal, a band pass filter 210 extracts a signal component, a band stop filter 212 extracts a noise component, an S/N comparator 220 calculates the S/N, a training control circuit 224 controls an automatic equalizer 208, the equalizer 208 converges an equalizing parameter and a discriminator 226 discriminates a data.