PURPOSE: To obtain excellent output waveforms, by forming the thin-wire shaped first and second magnetoresistance elements comprising thin ferromagnetic films on an insulating substrate, and setting an angle formed by the longitudinal directions of both magnetoresistance elements at a specified angle.

CONSTITUTION: In a magnetism detecting device, a rotary body 1 is constituted of a gear comprising a magnetic body. A rotation detector 2 is fixed at a place having a specified positional relationship so as to face the gear. The rotation detector 2 is formed by evaporating a thin film of an Ni-Co alloy on an insulating substrate 3 comprising, e.g., a substrate of a silicon oxide film, etching thin- wire shaped magnetoresistance elements 4 and 5, and by assembling a bridge. At this time, the detector is manufactured so that an angle formed by the longitudinal direction of the first magnetoresistance element 4 and the longitudinal direction of the second magnetoresistance element 5 is 120°. Thus, the resistance against the magnetic fields of noise is increased, and the excellent output waveforms can be obtained.