1. An image processing apparatus for performing predetermined processing with respect to an image on a document, the image on the document having a character/line portion, a gradation image portion, and a background portion which is distinct from the character/line portion and the gradation image portion, the image processing apparatus comprising: scanning means for repeatedly scanning the image on the document in a main scanning direction, with the scanning means being shifted in a sub scanning direction between the repeated scans in the scanning direction, thereby outputting image data including a plurality of values which represent image densities of the image on the document; binary image data preparation means for preparing binary image data which expresses the image data as white pixel data when the value of the image data is smaller than a predetermined value and which expresses the image data as black pixel data when the value of the image data is equal to or larger than the predetermined value; expansion image data preparation means for preparing expansion image data by converting a predetermined number of white pixel data adjacent to the black pixel data into black pixel data on the basis of the binary image data so as to convert the white pixel data between the black pixel data corresponding to the gradation image portion to black pixel data; and identifying means for identifying a portion of the image data as not belonging to the character/line portion, by checking a consecutive number of black or white pixel data of the expansion image data in the main scanning direction or the sub scanning direction.