An anti-theft tag which when attached to an article containing a conductive surface properly transmits a signal to a control device to generate an alarm if the article has been taken without permission. The anti-theft tag includes a resonance circuit and a soft magnetic material disposed between an attaching face of the article and the resonance circuit. The soft magnetic material allows the anti-theft tag to provide a resonance frequency which is independent of the material the anti-theft tag is attached to. The soft magnetic material may be composed of, for example, any one of an amorphous alloy, PERMALLOY, electromagnetic soft iron, silicon steel sheet, sendust alloy, Fe--Al alloy, and soft magnetic ferrite. The soft magnetic material may also be a composite material of soft magnetic material flakes and a plastic.