PURPOSE: To make it possible to detect the accurate azimuth at all times, by providing a demagnetizing coil in the vicinity of a geomagnetism sensor, and providing a power source device which supplies AC to said coil.

CONSTITUTION: The azimuth detecting device comprises the geomagnetism sensor 1, the loop shaped demagnetizing coil 2 which is arranged around the sensor 1, and the power source device 3 which supplied the AC to said coil. The sensor 1 and the coil 2 are mounted, e.g. in the inside of the roof of a body. When an automobile passes a railroad crossing and the like, a power source switch 33 of an operating part B is turned ON. At the same time, the AC, which is supplied to the demagnetizing coil 2 through a battery 30 in a power supply part A, a DC-AC converter 31, and attenuator 32, s slowly decreased from the maximum value by an adjusting dial, and the magnetized magnetic field of the body is demagnetized. Therefore the geomagnetic sensor senses the geomagnetism without the effect of disturbance, and the accurate azimuth detection is performed.