A toner image is formed on a sheet based on image data by an image forming portion, and a plurality of sheets on which the toner image is formed is bound together by a stapleless binding unit including a pair of tooth-like members, each having a concave and convex portion. Then, the image data is corrected by a correction portion so that a ratio of toner within a part of the sheet to be subjected to binding processing by the stapleless binding unit is equal to or smaller than a predetermined value.