A theft checking system that improves detection precision of theft checking systems. A micrometer samples data of received signals for three periods of a swept signal. A signal analyzer confirms signals that are regarded as resonance signals occurring in each of the first half and the second half of the first period, ignoring noise signals, and determines only whether similar signals corresponding to the resonance signals are present in each of the following two periods. A detection circuit recognizes the presence of the resonance signals and thereby recognizes and notifies passage of a tagged article.