PURPOSE: To correct the target altitude in accordance with the change in sea conditions or the flying speed of a flying object so as to realize the very low altitude flight of the flying object.

CONSTITUTION: A sensor 2 for altitude above sea level and a vertical acceleration sensor 2 are mounted on the flying body. A sea surface wave calculation means 6 calculates the sea surface wave based on the output of the sensor 3 for altitude above sea level and the output of the vertical acceleration sensor 3. A target setting means 7 sets target altitude based on the sea surface wave calculated by the sea surface wave calculation means 6. A calculation means 8 for average altitude above sea level calculates the average above altitude above sea level including a low frequency sea surface wave based on the output of the sensor 2 for altitude above sea level and the output of the vertical acceleration sensor 3. An automatic pilot device controls the altitude of the flying body so that the average altitude above sea level including the low frequency surface wave calculated by the calculation means 8 for average altitude above sea level coincides with target altitude. The flying speed of the flying body is measured and the target altitude of the target setting means 7 can be corrected so that it becomes large at the time of high speed by the measured sped value.