It is related to a mode S secondary surveillance radar system for carrying out a target (aircraft) surveillance with a high reliability, where a surveillance processor 39 produces a detection report of a acquisition surveillance by a scan of an antenna thereafter by adopting the mode A code information with coinciding code data among mode A code information acquired by scans of a plurality of times. In this way, the efficient utilization of RF channels during the roll-call period can be maintained while improving the reliability of a target detection report.