PURPOSE: To make it possible to accurately locate the north even under the vibratory condition with the use of a simple construction using a simple compensating system but not an inertia device.

CONSTITUTION: In a sensor unit 1, Xo and Yo coordinates are made to have the north an azimuth angle ψ on a horizontal plane, and an X axis is made to have the Xo axis and θ whereas a Y axis is made to have the Yo axis and an angle of inclination . The output of an angular sensor 11X is given by the equation containing the vertical and horizontal components of the axis of revolutions of the earth, and the output of an angular sensor 11Y is given. On the other hand, the output aX of an X axis acceleration sensor 12X is divided by an acceleration g with the use of a multiplier to determine the angle of inclination θ and is multiplied by the vertical component ΩV. These are calculated to determine the angular velocity on the Xo axis so that the angular velocity is integrated by an integrator 23 to determine an angle Ao. Then, the influences from the vibrations are eliminated and compensated by an adder 25. Likewise, fo the Y axis, compensation is made by an adder 30 to determine the angle Bo so that the value ψ for each second can be continuously determined by the calculation.