A crease detecting unit 13 detects a crease in a document image. A toner peeling detecting unit (a) detects as a toner peeling part a pixel area on the crease, the pixel area (a1) having a width of the predetermined number of pixels in a direction perpendicular to the crease, (a2) having a background color, and (a3) of which both sides are adjacent to at least two pixels in a direction perpendicular to the crease, the at least two pixels having a color other than the back ground color, and (b) corrects a pixel value of the toner peeling part on the basis of pixel values of at least two pixels adjacent to both sides of the pixel area.