PURPOSE: To apply precise takeoff and landing times to a radar information processor by connecting an aircraft takeoff/landing monitoring sensor to a transponder device mounted on an aircraft.

CONSTITUTION: The aircraft takeoff/landing monitoring sensor 9 for transmitting takeoff/landing information is connected to the secondary monitoring radar information processor provided with the transponder device 3 mounted of the aircraft to which a barometric altimeter 1 and a pilot sheet input device 2 are connected and a secondary monitoring radar 4 outputting a question wave to the transponder device 3 and receiving a response wave from the device 3. The transponder device 3 receives takeoff/landing information from the aircraft takeoff/landing monitoring sensor 9 and adds information indicating that the aircraft is flying or on the ground to the response of the radar 4, so that an air traffic control radar information processor 6 can decide the takeoff/ landing state of the aircraft easily.