PURPOSE: To detect the position of a transponder from the original station with high responsibility by receiving the response signal of the transponder corresponding to the inquiry signal from a secondary radar and calculating, based on the both signal and the relative position between the radar and the original station.

CONSTITUTION: The inquiry signal from a secondary radar SSR 3 is sent to the transponder of an airplane 1, and also sent to a transponder position confirmation device 7. The response signal of the airplane 1 to that is sent to the radar SSR 3 and the device 7. The device 7 detects each timing of the inquiry signal and the response signal, and based on both timings, calculates the distance from the radar SSR 3 through the airplane to the device 7 and azimuth of the radar SSR 3. From these and distance and azimuth angle between the radar SSR 3 and the device 7 measured in advance, distance and azimuth angle between the device 7 and the airplane 1 is found by compensation-calculation.