An image forming system includes a digital copy machine to form an image on a group of sheets from an image of a group of documents, and a staple sorter sorting and discharging copied sheets from the digital copy machine. According to the staple sorter, control is provided so that a sheet of a long left period of time is transported from the bin to a nonsort tray when all of a plurality of bins provided in the staple sorter are used during a print operation. The left time of a sheet is monitored even when a print operation is not carried out. When the left time exceeds a predetermined time, control is provided so that the sheet left in the bin is transferred to the nonsort tray. As a result, a sheet discharge device is provided improved in operability of sheet discharge.