A post-processing device includes a processing tray on which sheets are to be stacked, a transport portion for transporting, a sheet introduced through an introduction port, a turn-aside portion for turning aside the sheet to delay arrival of the sheet at the processing tray, and a delivery mechanism for delivering the sheets stacked on the processing tray to a delivery tray on a boundary between copies. When a remaining number of sheets of one copy unstacked on the processing tray exceeds a maximum number of stacked sheets that is an amount of sheets stackable on the processing tray, the delivery mechanism delivers a sheet stack of the one copy a plurality of times so as to avoid delivery of only one sheet on a last page of the one copy, and the turn-aside portion turns aside a sheet on a first page to be stacked after delivering the sheet bundle.