An image forming system has a color scanner; an image processing part for processing image data obtained by reading a document by use of the color scanner; and an image forming part for forming an image on the basis of image data processed by the image processing part, wherein the color scanner reads a first original which includes a portion of a character or diagram having a first color, and a portion which is marked in a second color different from the first color so as to cover information of the character or diagram, and the image processing part edits image data of the marked portion of image data of the read first original. The editing process includes a process for rectangularizing the marked portion, and a process for connecting the rectangularized regions when the rectangularized regions are close to each other.