A GPS measuring unit receives a signal from a GPS satellite and outputs GPS measurement data including at least orientation of an object. An angular velocity sensor outputs angular velocity of the object. An offset value computing unit estimates a running condition of the object on the basis of the measurement data and the angular velocity. The offset value computing unit sequentially derives a temporary offset value of the angular velocity sensor in accordance with the estimated running condition of the object. The offset value computing unit derives an offset value of the angular velocity sensor by executing statistical process on the temporary offset value of the angular velocity sensor. A forgetting factor in the statistical process is changed according to the running condition of the object.