Abstract: A sensor capable of reliably detecting a pulse voltage generated by a magnetic material by means of electromagnetic induction and a security tag using the sensor, which produces an alarming sound and causes a security gate to produce another alarming signal to thereby specify a product brought out illegally while preventing an erroneous operation of the security tag in usually handling a product, are provided. A pulse generating member of magnetic material which is inserted into a hollow tube in a free movement state generates a pulse voltage in an A.C. magnetic field. The pulse voltage is derived by a pick-up coil wound on the hollow tube as a sensor output. The pulse voltage thus derived activates an operation control circuit to actuate an alarm device and an alarm device of the security gate is operated by the pulse voltage or its high harmonics or a pulse control signal generated by the operation control circuit or its high harmonics.