PURPOSE: To always keep the amplitude of the output of a filter constant by monitoring the amplitude of an output signal by a level detection circuit, supplying information representing the fact that the level of the amplitude exceeds an ordinary level by a control circuit to a coefficient setting circuit when it occurs, and holding a coefficient value at that time.

CONSTITUTION: A signal inputted to an input terminal 1 is equalized with a transversal filter 2, and the output is inputted to the coefficient setting circuit 3 and the level detection circuit 5. At the level detection circuit 5, the level comparison of the output signal with a threshold value higher than ordinary mean amplitude is performed, and when the level of the output signal exceeds the threshold value, a level detection signal is outputted to the control circuit 6. At the control circuit 6, a coefficient setting interruption instruction is outputted to the coefficient setting circuit 3 based on the level detection signal. At the coefficient setting circuit 3, the holding operation of the coefficient value is performed based on the coefficient setting interruption instruction. As a result, the increment of the signal level of the output can be suppressed with a means to hold a coefficient before interruption.