PURPOSE: To attain the application of the same equalizer to different transmission speeds by varying the sample frequency of a root-f equalizer.

CONSTITUTION: The frequency characteristic of a root-f filter RTFEQ is moved in parallel on a frequency axis and turned into the characteristic adaptive to the transmission speed. Then the desired cut-off frequency of a loopback noise eliminating pre-filter is increased or reduced in response to a fact that the sample frequency of the filter RTFEQ is increased or decreased as the transmission speed is increased or reduced. In addition,the flat gain of an amplifier AGCMP is increased or reduced in response to the sample frequency of the filter RTFEQ since the line loss is increased or reduced in a frequency area as the transmission speed is increased or reduced. In such constitution, an equalizer can secure the frequency characteristic in accordance with the signals of different transmission speeds.