A magnetic detecting device according to an embodiment includes: a magnetic sensor head in which a plurality of magnetic sensing elements including a first magnetic sensing element and a second magnetic sensing element are arranged; and an environmental magnetism corrector. The plurality of magnetic sensing elements is arranged in one line over a length that is greater than a width of a sheet, in a direction that is orthogonal to a movement direction of the sheet. The environmental magnetism corrector corrects a first output signal output by the first magnetic sensing element, based on a second output signal output by the second magnetic sensing element. The magnetic sensor head has a sheet passage area that faces a sheet that passes over the magnetic sensor head, the first magnetic sensing element is positioned inside the sheet passage area, and the second magnetic sensing element is positioned outside the sheet passage area.