1. A system for detecting an edge of an image, comprising: first edge detecting means for extracting a first edge of an original image using a first threshold; reproduction processing means, operatively connected to said first edge detecting means, for obtaining a reproduced image corresponding to the original image using the first edge; error extracting means, operatively connected to said reproduction processing means, for extracting an error between the original image and the reproduced image; second edge detecting means, operatively connected to said error extracting means, for detecting a second edge in the error extracted by said error extracting means using a second threshold smaller than the first threshold; and edge correcting means, operatively connected to said first edge detecting means and said second edge detecting means, for correcting said first edge by logically adding said first edge and said second edge to obtain a corrected edge in a corrected reproduced image.