To provide a correcting method of image distortion capable of performing skewness aberration correction with high precision by establishing appropriately ideal coordinate values without the use of complicated devices.

A photographed skewness correction image of digital data is acquired through camera-filming of skewness correction pictures having a central circle placed at the center and grid points arranged at an equal spacing, the positions of ideal grid points are decided from the ratio of the diameter of circle in the image concerned to the spacing of grid points, correction formula is derived on the basis of the positions of each photographed grid points and the positions of ideal grid points, and finally skewness aberration can be corrected with correction formula concerned.