Abstract: A theft checking system that improves precision in detecting a tag and prevents false notification. A first computer (S3) samples a waveform in a fixed length of time of a signal received by a reception antenna. A digital direct synthesizer (S4) divides the sampled waveform data of the fixed length of time and a reference signal pre-registered in a memory circuit of a calculating portion respectively into portions by equal pitch with respect to the time axis, multiplies the value of the reference signal at each point separately by the value at each point in the data corresponding to that point, and calculates the sum of the products. A matching circuit (S6) compares the calculated value with a predetermined value to detect a waveform that agrees with the reference oscillation. A tag (S7) actuates notifying means.