Adaptive equalizing filter of reduced electric power consumption for use in communication system

In an adaptive equalizing transversal filter for quantizing a transmission waveform deformed because of communication line characteristics, a control circuit is provided for inhibiting the updating of mutiplication coefficients when an error signal between an equalized value and a quantized expected value is small, so as to omit the mutiplication operation required for obtaining the mutiplication coefficients, thereby reducing power consumption.