PURPOSE: To guarantee the adaptive operation by selecting a difference signal which is opposite in polarity and whose absolute value is nearly equal to the previous values stored in a memory, summing the selected value with the existing value, applying the revision of coefficient with the sum and controlling the adaptive filter.

CONSTITUTION: The minimum value of a probability where a present sample and a sample before JT sec preceding are of opposite in the polarity and the absolute value is nearly equal and is a positive value hot being zero according to the characteristic of the eye pattern of a reception signal of a transmission line code including a binary-code system. Thus, a difference signal (=reception signal including residual inter code interference) is stored in a memory 10 corresponding to the symbol waveform to which each sample belongs and when a sample of opposite polarity and equal absolute value is extracted when it is received and added to the existing sample thereby causing the sum to be zero in the absence of inter-code interference and to be the residual inter-code interference itself in other cases. Thus, the residual inter-code interference is detected accurately, then the sum is used as an error signal to guarantee the adaptive operation of the adaptive filter 5.