Disclosed is a Mode S radar which specifies an aircraft by transmitting interrogations to an aircraft A equipped with a Mode S transponder, and by then receiving and decoding replies corresponding to these interrogations. In the Mode S radar, detection reports on the aircraft A are generated in a manner that: for a Mode A code necessary for generating detection reports, when identicalness between the Mode A codes obtained in a plurality of scans after the initial acquisition of the aircraft A has been found to exist, the Mode A code thus found identical is adopted without carrying out a Mode A code interrogation (UF="5") thereafter. Accordingly, the Mode S secondary surveillance capable of generating more highly reliable detection reports is achieved.