PURPOSE: To effectively judge even a small number of responses of small pulse width or amplitude by marking pulses whose pulse width and height are greater than other preset values smaller than normal pulse width and height, and calibrating them as candidates for pulse signals.

CONSTITUTION: Received response signals 21 of a secondary radar (SSR) are subjected to pulse calibration by a pulse width/amplitude calibrating circuit 22. For those received signals 21 whose pulse height and width become smaller than the normal pulse height V1 and pulse width t1 because of fluctuations, etc., a pulse width/amplitude rescue calibrating circuit 23 sets another pulse height V2 and pulse width t2 to perform pulse calibration, and outputs a rescue calibration output 25 provided with an effective marking signal 26. An effective pulse reinforcing circuit 27 ORs the detection output 24 of the circuit 22 and the output 25, and outputs the result as an effective pulse output 28. When the output corresponds to the period of rescue pulse, it is provided with a mark output 29 to show that the period of the output is that of rescue pulse. A response code judging circuit 30 analyzes a response code according to the output 28.