A waiting tray is provided in a sheet-conveying path and holds sheets if the sheets need to be post-processed. A processing tray receives sheets conveyed from the waiting tray. It may receive sheets conveyed through the sheet-conveying path and coming not via the waiting tray, before the sheets are post-processed. A conveying mechanism is provided, which causes the sheets to fall, due to gravity, from the waiting tray onto a processing tray. A sheet-aligning mechanism aligns the sheets on the processing tray, at their transverse edges and longitudinal edges. On the processing tray, the sheets are post-processed, forming a bundle. The conveying mechanism conveys the bundle of sheets to a storage tray. An opening is provided between the waiting tray and the processing tray and located at downstream of the waiting tray and processing tray.