To provide an anti-theft device with high reliability.

When an object T fitted to the anti-theft device approaches a gate and a tag main body 2 receives a signal sent from a transmitter, a controller built in the tag main body 2 places an alarm in operation to generate an alarm, thereby preventing the object T from being stolen. When the tag main body 2 is separated from the object T, a lever projects from a case, so the controller detects that and places the alarm in operation to generate an alarm. When a belt material 5 is cut while the lever is pressed by inserting a thin plate material between the tag main body 2 and object T or after the tag main body 2 is fixed to the object with an adhesive tape, both contactors provided on the tag main body 2 are not electrically connected to each other, so the controller detects that and places the alarm in operation to generate an alarm.