PURPOSE: To improve the output accuracy of an equalizer, by correcting a tap coefficient to only a small error of equalized output.

CONSTITUTION: An equalizer consists of an error generating circuit 1, an error amplitude calculating circuit 2, a control circuit 3 and a tap coefficient correcting circuit 4. The circuit 1 generates errors Er, Ei from the difference between equalized outputs yr, yi and reference values rr, ri. The circuit 2 inputs the errors Er, Ei and outputs the sum of square. The circuit 3 compares a reference value Mg determining the propriety of the tap coefficient correction with the output of the circuit 2, and only when the output of the circuit 2 is smaller, a trigger signal is given to the circuit 4 for the correction of the tap coefficient.