An electrical pulse current is supplied to an amorphous wire from a pulse generator, an alternate current voltage whose magnitude is in response to an external magnetic field induced at both ends of a detecting coil wound around the amorphous wire is generated, a positive direct current is applied from a positive power supply by superimposing to the amorphous wire as a bias current so as to produce a bias of magnetization within the amorphous wire and the occurrence of pulse noises is restrained, so as to make it possible to perform high-sensitivity magnetic field detection.