In a sheet folding device 1 provided with a folding part 70 for folding a sheet S carried into the device, a discharge part 80 for discharging a sheet S already subjected to folding processing, and a discharge tray 74 for receiving a sheet S discharged from the discharge part 80, a holding member 5 for holding a discharged sheet S from above and an arm 4 disposed above the discharge tray 74 and also adjacently to the discharge part 80 and so provided as to project in a sheet discharge direction are provided. The holding member 5 is supported at the arm 4 in such a manner as to be rotatable upward and downward, and also rotatable to a position above the bottom end of the arm 4.