Magnetism detecting device and electronic azimuth meter using the same

A voltage generating section (11) applies first and second voltages having inverted polarities to a sensor section (12). The sensor section (12) outputs a voltage value corresponding to a change in geomagnetism. A bias magnetic field generating section (16) applies an alternating current bias magnetic field to the sensor section. A detecting section (13) detects voltages with respect to the first and second voltages when the alternating current bias magnetic field is applied in positive and negative directions. By taking the difference between the detected voltages, an offset voltage to be outputted from a magnetism detecting circuit (131,132) is eliminated.