PURPOSE: To suppress the distortion due to a hump to obtain a distortionless detection signal, by generating a voltage signal of a prescribed time width based on the rise part of the output of an AM detecting circuit and by slicing a signal obtained by adding this signal and the output of the detecting circuit.

CONSTITUTION: The receiving wave which has the amplitude modulated by a square wave has the band limited by a filter 2 and is converted to DC by a rectifying circuit 4. The output of the rectifying circuit 4 is inputted to not only an adder 11 but also a variation detecting circuit 8, and the rise part and the fall part are taken out as a positive pulse and a negative pulse respectively. A rectifying circuit 9 takes out a negative pulse signal indicating the fall part and sends it to a timer circuit 10. The timer circuit 10 generates a negative voltage signal of a prescribed width and applied it to the adder 11. The signal from the adder 11 is applied to a comparator 5, and a signal where the hump generated by band limiting of the filter 2 is suppressed is obtained as the output.