PURPOSE: To correct an error in direction detection easily and accurately only by turning a vehicle without the need to turn the vehicle to the north and east of terrestrial magnetism.

CONSTITUTION: The X axis 11 in a figure which shows the state of a magnetic field where a terrestrial magnetism sensor 6 is installed by vectors indicates the left-right direction of the vehicle, and the Y axis 12 indicates the front-rear direction of the vehicle. A track 16 shows a composite vector H generated when the vehicle turns. The vehicle makes one turn to collect data at respective points of a circular track 16 previously, and (y) coordinates Ay and By of intersections A and B of l1 and the circular track 16 and (x) coordinates Cx and Dx of intersections C and D of l2 and the circular track 16 are calculated to solve (Ay+ By)/2 and (Cx+Dx)/2, so that the results are the (x) and (y) components of a vehicle magnetism vector K because the terminal point of the vehicle magnetism vector K is the center of the circular track 16. This vehicle magnetism vector K is subtracted from the composite vector H to obtain a terrestrial magnetism vector E, making an accurate correction.