To improve stacking performance in stacking recording material bundles having a larger thickness area and a smaller thickness area.

In performing binding processing by a binding device 500, a sheet bundle stacking section 70 is elevated, the top sheet bundle out of a plurality of stacked sheet bundles is pushed against a unit 510. Thus, an area having a larger thickness in the sheet bundle (area to which binding processing is executed) is pressed, and a thickness of each sheet bundle is reduced. And in this case, collapse of the stacked sheet bundles is hard to occur. Further, the number of sheet bundles that can be stacked can be increased.