PURPOSE: To improve the performance of a magnetic compass, by providing loop coils in the attaching frame of the magnetic compass, forming a constant magnetic field by the loop coils, thereby reducing errors.

CONSTITUTION: In right and left electrically heated windhields 20R and 20L, electrode plates 16R and 16L, resistors 17R and 17L, and grounding electrode plates 18R and 18L are provided. A magnetic compass 10 is arranged at the central position between the right and left electrically heated windshields 20R and 20L. right and left loop coils 11R and 11L correct the strain-magnetic-field vector yielded in the magnetic compass 10 by correcting-magnetic-field vector in the reverse direction. The exciting currents from relays k1 and k2 for power source control for the electrically heated windshields flow throgh the loop coils 11R and 11L.