A DME ground apparatus comprising: an aerial unit adapted to receive an interrogation signal from an aircraft; a transponder unit adapted to receive the interrogation signal from the aerial unit, generate a response signal constituted by pulse pairs in response to the interrogation signal and transmit the response signal to the aircraft; and a monitor control unit adapted to monitor and control the transponder unit, characterized in that the transponder unit has a digitizing unit adapted to perform analog-to-digital conversion on the interrogation signal received by the aerial unit to generate a digital signal, a pulse detection unit adapted to detect pulses from the digital signal generated by the digitizing unit, a response signal pulse transmitting unit adapted to generate the response signal in response to the interrogation signal detected by the pulse detection unit and transmit the pulses of the response signal at a predetermined transmission rate, and a reception level control unit adapted to detect the transmission rate of the pulses transmitted from the response signal pulse transmitting unit and control a threshold of the reception level of the pulse detection unit arranged in the transponder unit in accordance with the transmission rate when the transmission rate increases, and the transponder unit is adapted to variably control the threshold of the reception level in accordance with a rate at which to transmit the pulse-pairs constituting the response signal, so as to process only interrogation signals received at a reception level higher than the reception level threshold and to control the rate at which to transmit the pulse-pairs such that an upper limit is not exceeded. an aerial unit adapted to receive an interrogation signal from an aircraft; a transponder unit adapted to receive the interrogation signal from the aerial unit, generate a response signal constituted by pulse pairs in response to the interrogation signal and transmit the response signal to the aircraft; and a monitor control unit adapted to monitor and control the transponder unit, characterized in that the transponder unit has a digitizing unit adapted to perform analog-to-digital conversion on the interrogation signal received by the aerial unit to generate a digital signal, a pulse detection unit adapted to detect pulses from the digital signal generated by the digitizing unit, a response signal pulse transmitting unit adapted to generate the response signal in response to the interrogation signal detected by the pulse detection unit and transmit the pulses of the response signal at a predetermined transmission rate, and a reception level control unit adapted to detect the transmission rate of the pulses transmitted from the response signal pulse transmitting unit and control a threshold of the reception level of the pulse detection unit arranged in the transponder unit in accordance with the transmission rate when the transmission rate increases, and the transponder unit is adapted to variably control the threshold of the reception level in accordance with a rate at which to transmit the pulse-pairs constituting the response signal, so as to process only interrogation signals received at a reception level higher than the reception level threshold and to control the rate at which to transmit the pulse-pairs such that an upper limit is not exceeded.