A first discharging portion that discharges a sheet received from one of image forming apparatus and a second discharging portion discharges a sheet received from another image forming apparatus are disposed opposite each other to stack the sheets discharged in a common processing tray. A controller controls the first and second discharging portions when the sheets are continuously discharged by the first and second discharging portions, controls a timing when the sheets are discharged by the first discharging portion and the second discharging portion to the common processing tray such that a leading edge of the sheet discharged from one of the discharging portions abuts on a sheet surface in the downstream of a discharging direction below a leading edge of the sheet discharged from the other discharging portion.