A transmitter (13) operates in an immediate scan after an initial acquisition or detection of an altered aircraft ID by a changed flight status, to transmit to an aircraft a sequence of interrogation signals requesting an aircraft ID transmission, a verifier (17) operates upon acquisition of a reply signal including an aircraft ID transmitted from a transponder on the aircraft, to store in a memory (17a) a mode S address assigned to the aircraft and the aircraft ID in an associating manner, and upon an occurrence of a storage of aircraft IDs associated with the mode S address in the memory (17a), to determine whether or not the aircraft ID is correct, depending on whether or not the aircraft IDs have a match therein, and a report generator (18) operates upon a determination for the aircraft ID to be correct, to prepare a target report using the aircraft ID.