PURPOSE: To prevent a feedback loop from oscillating even when the amount of interference between codes increases by releasing the feedback loop which includes a backward equalizer for a specific period when the tap gain of a forward equalizer decreases below a specific value.

CONSTITUTION: When the amount of intercode interference of an adaptive decision feedback type automatic equalizer increases, all tap gains of a transversal filter constituting the forward equalizer 1 are suppressed, so the values are smaller than the specific value and a comparator 11 generates a "1"-level signal, which is supplied to a switch circuit 10 through a delay circuit 12. The circuits 10 opens terminals 10a and 10b to release the feedback circuit including the backward equalizer 2, preventing the oscillation. When the equalizer recovers to the normal state, the tap gains of the equalizer 1 exceed the specific value and the comparator 11 outputs "0", but the circuit 12 outputs the "1"-level signal for a specific time. Consequently, the circuit 10 is closed a specific time after the equalizer recovers to the normal state, thus attaining recovery to the normal state without causing malfunction.