To accurately compensate influence of earth flatness of an earth sensor and influence of orbit eccentricity.

A GPS antenna 5 receives signals from a GPS satellite. A GPS receiver 6 calculates position using a navigation information obtained by decoding a navigation message included in a pseudo range and a pseudo range-sheet and signals, which are obtained when the GPS antenna 5 locks a received signal. A calculation part 2 calculates a distance to an earth center based upon position information output by the GPS receiver 6, a compensation part 7 calculates a compensation value of an earth sensor 3 in accordance with the distance to the earth center output by the calculation part 2, and then the compensation value is added to an output of the earth sensor 3 by an adder 4.