PURPOSE: To minimize the generation of code errors by adjusting an identification level to an optimum value automatically in response to the quantity of generation of reception code errors.

CONSTITUTION: The circuit is provided with a code error detection section 4 being an error generation rate arithmetic means and an identification level control section 7 adjusting the level generated by an identification level adjustment section 6 in response to the increase/decrease in the generation rate calculated by a code error count section 5. Then the deviation of the identification level from the optimum value due to the change in the characteristic of a circuit component attended with the change in the secular characteristic change of the circuit component and the environmental condition change is corrected automatically by applying feedback adjustment automatically to the identification level at the identification recovery section 2 in response to the change in the code error rate of an output signal. Thus, the generation of code errors is minimized.