A sheet processing device includes an exit tray and a sheet alignment mechanism. On the exit tray, ejected sheets or ejected sheet sheaves subjected to stapling are stacked. When at least a sheet is ejected, the sheet alignment mechanism aligns the sheets in a manner to be in contact with and catch opposite ends of the sheets stacked on the exit tray from a base end side of the exit tray. When a sheet sheaf is ejected, the sheet alignment mechanism comes in contact with a plurality of points on a topmost surface of the sheet sheaves stacked on the exit tray to detect a height difference between the plurality of points.