To provide a magnetism detecting device capable of correctly detecting a fine local magnetic pattern recorded on a magnetic card, a bank bill.

A first and a second wire-like magnetic impedance elements 11, 12 are provided in line in the lengthwise direction, the lengthwise direction is arranged so as to be perpendicular to a bank bill 14, and magnetic flux is acted only on the first magnetic impedance element 11. After detecting the output signals of the first and second magnetic impedance elements 11, 12 by respective wave detecting circuits 15, 16, they are supplied to the positive and negative input terminals of a differential amplifier 17. Because the external noise magnetic flux similarly acts on both magnetic impedance elements 11, 12, and by taking differential it can be cancelledly removed, the fine local magnetic flux can be correctly detected without being influenced from noise.