To provide a shoplifting preventing device in which the passage of a resonant tag and automatic ringing tag applied to merchandise can be detected without any malfunction by using an antenna provided at one doorway.

In a shoplifting preventing device 1, different frequencies are set in a first signal for operating an automatic ringing tag 8 and a sweep signal for detecting a resonant tag 6, and each signal is outputted by time-division to a first transmitting antenna 10 and a second transmitting antenna 12 provided at the same doorway, and a radio wave for detecting each tag is radiated from each transmitting antenna 10 and 12. Also, a received signal from a receiving antenna 14 which receives the sweep signal for detecting the resonant tag is synchronization detected by a receiving part 40, and the intrusion of the resonant tag 6 to the doorway (in other words, the shoplifting of merchandise to which this tag is applied) is judged from the detected signal (a difference signal). As a result, the tag in each system can be exactly detected at the same doorway, and a shoplifting preventing system using the two systems can be easily constructed.