There is provided an antitheft device and an antitheft system capable of reliably detecting theft via a simple installation method. The antitheft device includes a receiver 12 that receives a radio wave of a predetermined frequency, and a microcomputer (controller) 18 that determines whether or not the antitheft device is located at a position where it is impossible to detect a radio wave of a predetermined frequency by the receiver 12. The microcomputer (controller) 18 performs an alarming operation, upon determining that the antitheft device is located at a position where it is impossible to detect a monitoring radio wave WW of a predetermined frequency by the receiver 12. Accordingly, it suffices to attach the antitheft device 10 to a managed object MO such as a commercial item being monitored, and therefore it is not necessary to extend a cable from a sensor part attached to the commercial item.