Image matching on a pair of image data about images which are shot at different shooting positions is performed, and a point corresponding to the coordinates of a distance measurement point on the image shown by one image data of the pair is searched through the image shown by the other image data of the pair. The value of a parameter showing the attitude of an airplane (2) is corrected in such a way that the difference between the coordinates of the distance measurement point on the image shown by the other image data of the pair and the coordinates of the corresponding point which is searched for via the image matching becomes small, to estimate the attitude of the airplane (2).