To transmit high-speed data by means of optional cable length by changing a frequency characteristic in accordance with the cable length of a detected coaxial cable so as to automatically equalize a transmission signal.

A cable length detecting circuit 2 wave-detects a part of a frequency component of an input signal and obtains its wave-detection voltage. The wave-detection voltage is transmitted to a bias voltage generating circuit 4. In the bias voltage generating circuit 4, the voltage required for the bias of an equalizing circuit 3 is generated from the wave-detection voltage. The bias voltage is transmitted to the equalizing circuit 3. The equalizing circuit 3 is provided with a circuit where the frequency characteristic is changed in accordance with the control voltage and an output from the cable length detecting circuit 2 is inputted to the circuit 3 as the control voltage. Concretely speaking, the bias voltage corresponding to the cable length of the coaxial cable from the bias voltage generating circuit 4 is added so as to equalize a reception signal. Then, it is amplified by an amplifier 5 and added to a quantization feed back circuit 6.