PURPOSE: To set the optimum tap weight by providing an automatic gain controller between the output of an automatic equalizer and a deciding device in order to correct the attenuation of the data output.

CONSTITUTION: The input signal given from an input terminal 1 of an automatic equalizing device is supplied to an automatic equalizer 3. Then an equalized signal is supplied to a deciding circuit 4 from an output terminal 6 via an automatic gain control circuit 15. The circuit 4 decides the closest transmission data point and delivers it to a decision output terminal 2 and also to a terminal at one side of a subtractor 5. While the output of the equalizer 3 is supplied to the other terminal of the subtractor 5. Then an error signal given from the subtractor 5 is applied to a tap weight controller of the equalizer 3. The attenuation of the data output of the equalizer 3 is corrected by the circuit 15. Thus the characteristics of the automatic equalizing device are improved in the case of large circuit distortions.