A magnetic-field detecting device includes a pair of magneto-sensors including respective magnetism sensing portions that sense magnetism, and respective coils sensing changes of magnetic fluxes in the magnetism sensing portions, and an elongate connecting member cooperating with the magnetism sensing portions to constitute a magnetic circuit. A magnetism sensing direction of the magnetism sensing portions coincides with a longitudinal direction of the connecting member to an extent that permits the coils to equally sense a magnetic field applied to the coils the connecting member being formed of a magnetic material having a relative magnetic permeability of at least 100, a magnetic material having a relative magnetic permeability which is at least 1/100 of that of a magnetic material of the magnetism sensing portions, or the same magnetic material as the magnetism sensing portions the magnetic-field sensor measuring the magnetism on the basis of a difference between outputs of the coils.