PURPOSE: To make it possible to measure accurate azimuth by obtaining signals of two components corresponding to magnetism by a magnetism by a magnetic sensor, giving the signals to adders or subtractors together with the outputs of variable power sources, and removing the effect of erroneous magnetic fields.

CONSTITUTION: DC signals Uu and Uv are obtained by amplifying two signals eu and ev which are generated by the magnetic sensor which is mounted on a moving body in response to the applied magnetism. Said signals are given to one input terminal of each of two adders. The outputs of the variable power source are applied to the other input terminals. Thus the effect of the erroneous magnetic field is removed. For example, since the so called erroneous magnetic fields which are generated by the moving body and the like is generally acted on the magnetic sensor 1 in addition to terrestrial magntism, said effect is applied on the generated signal eu and ev. Therefore, the outputs of the variable power source Vu and Vv are acted in the adder 30 and 31 after the signals pass through amplifiers 20 and 21 in order to remove said effect, and signals Eu and Ev which are related only to the terrestrial magnetism can be simply obtained.