PURPOSE: To facilitate waveform equalization from the shortest length to the longest length of specific coaxial cable length by adding a direct circuit of a signal peak detector and a switch.

CONSTITUTION: The detecting circuit 1 detects whether or not the length L of a coaxial cable to the input terminal 10 of an equalizing amplification part 20 is longer or shorter than constant length L0 and the switch 2 turns on and off the connection between the input terminal and output terminal of the equalizing amplification part 20 with a control signal (c); when the detecting circuit 1 decides that the length L of said coaxial cable is shorter than the constant length L0, the switch 2 is turned on to directly connect the input terminal and output terminal of the equalizing amplification part 20 and a CMI signal at the input terminal of the equalizing amplification part 20 is outputted to a trailing-stage circuit as it is and converted into a unipolar signal. When the cable length is the shortest, waveform deterioration such as an overshoot and an unnecessary noise are caused to eliminate such a phenomenon that what is called EYE for checking whether a transmitting waveform is superior or inferior blurs.