PURPOSE: To detect the correct bearing by updating the offset value appropriately and excluding the effect of the drift of the offset even if a vehicle continues to run for long time.
CONSTITUTION: The bearing of a vehicle is detected by correcting the output of a gyroscope 1 with an offset value which is stored in an offset memory 11. When the vehicle continuously runs for specified time or longer, a position detecting part 3 obtains the first bearing-change amount based on the bearing information which is imparted from a GPS receiver 9. In the position detecting part 3, the value after the offset correction for the outputs of the gyroscope 1 during a period corresponding to the first bearing-change amount are accumulated. The accumulated value is made to be the second bearing-change amount. Furthermore, the position detecting part 3 computes the new offset value based on the first bearing-change amount and the second bearing-change amount and stores the new offset value into the offset memory 41.