To provide a sheet discharge mechanism free to immediately start interrupting motion and easy to fetch a sheet discharged by the interrupting motion in discharging the sheet concerning the interrupting motion on a sheet aftertreatment device furnished with a plurality of sheet discharge trays free to vertically move and not furnished with a non-sort tray.

A sub-tray 30 capable of taking both postures of a storing posture shown in a broken line and a developing posture shown in a continuous line is provided between a sheet discharge port 24 and the sheet discharge tray 25. The sub-tray 30 holds the storing posture in the middle of normal sheet discharge motion and changes the posture to the developing posture by a solenoid 33 to be actuated by a control device after the sheet concerning the discharge motion at a point of time when an interrupting command is issued. The sheet concerning the interrupting motion is discharged so that a head end part in the discharging direction is placed on a previously discharged sheet and a rear end part is placed on the sub-tray 30, and consequently, it is easy to discriminate it from the sheets discharged by the normal motion and to separate it from them, too.