PURPOSE: To execute navigation with high accuracy by operating a prescribed expression by using an output voltage of an earth magnetism sensor, discriminating it to be an azimuth error caused by magnetization of a car body, when said voltage exceeds a prescribed value, issuing an alarm, calculating a new offset by magnetization, and calculating an exact vehicle azimuth.

CONSTITUTION: A magnetic field by magnetization of a vehicle is added vectorially to output voltages Vx, Vy of earth magnetism sensors placed orthogonal to each other on the vehicle. When the vehicle rotates by 360°, Vx and Vy draw a circular locus, and in the Vx and Vy coordinate system, the center of a circle exists at offsets vxn, vyn and a radius KB of the circle is proportional to the magnitude of the horizontal component of the earth magnetism B. Even if a vehicle azimuth is varied, if magnetization M is constant, the radius KB is constant. When the magnetization is varied, the offset moves, and a circle C also moves. In this state, from the circular locus after one rotation of the vehicle, the offsets vxn, vyn are calculated and (Vx-vxn)2+(Vy-vyn)2 is calculated at a prescribed timing. When it exceeds the square of the radius of the circle C, an alarm is issued, and after they have been updated to new offsets vxn+1, vyn+1, an exact azimuth θ is calculated. According to this constitution, navigation can be executed with high accuracy.