PURPOSE: To provide a detector which is simplified and detects a label for prevention of shoplifting of commodities.

CONSTITUTION: A resonance circuit having a resonance frequency in the MHz band is provided, and pairs of transmission and reception antennas 8 to 13 which are fixed to passages 5 to 7 to be monitored are provided. Transmission antennas 8, 10, and 12 radiate electromagnetic waves whose frequencies exceed a prescribed resonance frequency of the label and are varied in variance cycles. Variance cycles are synchronized with one another. A reception circuit which detects the label is connected to reception antennas 9, 11, and 13. For the purpose of simplifying the detector, HF oscillation frequencies radiated as electromagnetic waves from transmission antennas are generated by HF oscillators 14 to 16 which are connected to transmission antennas and are shifted from the center, and a synchronizing signal having a frequency in 8 long wave band which is generated by a central unit 22 and is supplied to HF oscillators is used to synchronize the variance cycles to one another.