1. An image processing method for performing a processing of a line sensor image which is continuously taken by a line sensor installed on a flight vehicle, comprising: calculating first coordinates transformation information between an image coordinates system and a three-dimensional reference coordinates system for each line of the line sensor image; searching the most suitable line on the line sensor image corresponding to a position whose three-dimensional position is known in the three-dimensional reference coordinates system as known position, based on information of the known position, information about the three-dimensional position and the speed of the flight vehicle for each line of the line sensor image, and the first coordinates transformation information; and searching a position of a pixel on the line sensor image corresponding to the known position, based on information about the most suitable line, the information about the three-dimensional position and the speed of the flight vehicle for each line of the line sensor image, and the first coordinates transformation information.