A posture angle processing apparatus includes a magnetic sensor, a gravitational acceleration sensor, a position information acquisition device, and a dip angle database device wherein a correlation of the absolute location and a geomagnetic dip angle at the corresponding point is stored. Based on the absolute location of the current point, a dip angle estimation and determination device reads the geomagnetic dip angle at the current point, calculates an angle formed by a horizontal plane and the magnetic vector, compares the angle formed by the magnetic vector and the horizontal plane with the geomagnetic dip angle of the current point, and outputs either a true signal or a false signal depending on the error between the angle and the geomagnetic dip angle.