1. In an image processing system for reconstructing an image formed of plural lines from synthetic aperture radar's data, a method of synthetic aperture radar image reconstructing comprising the steps of generating a point image pattern in correspondence to each point of an original image, and performing fast Fourier transformation for the generated point image pattern and image data for one line, wherein said image has n lines and x lines of said n lines are processed, x being less than n, said processing steps being implemented for lines spaced by a predetermined plural number of lines.