To provide a sheet postprocessing device capable of space-saving in response to a thin sheet, without reducing aligning accuracy of a sheet bundle.

A sheet postprocessing device includes a sheet registering means having a staple registering tray, a center binding means having a staple center binding tray, and a center holding means. The staple registering tray and the staple center binding tray are arranged in this order downward from above one inclined face, and the center folding means is arranged under the staple center binding tray. The sheet postprocessing device includes an upper side guide plate for forming a sheet bundle passage together with the staple center binding tray, and a passage deforming means for forming a center folding means supply passage for supplying a center-bound sheet bundle of passing through the sheet bundle passage to the center folding means by deforming a lower end part of the sheet bundle passage to the center folding means side.