Patent ID: 8851464

Claim:
A sheet post-processing device that folds a sheet output from an image forming device at a predetermined target fold position, the sheet post-processing device comprising: a first rotating body pair configured to convey the sheet; a second rotating body pair disposed along a sheet conveyance path and further downstream than the first rotating body pair in a sheet conveyance direction; a push unit configured to move a push member in a direction transverse to the sheet conveyance path so that, while the sheet is being conveyed, the push member comes into contact with the sheet and pushes the sheet into a nip of the second rotating body pair, the push member being disposed opposite the second rotating body pair across the sheet conveyance path; and a control unit configured to control the push unit so that the pushing operation of the push member is performed at a timing preset according to the target fold position; wherein: a movement speed at which the push member is moved during the pushing operation is set to be faster than a conveyance speed at which the first rotating body pair conveys the sheet; and the timing is preset so that: the pushing operation is initiated such that, when the push member comes into contact with the sheet, the target fold position is located a predetermined distance downstream, in the sheet conveyance direction, from a contact position where the push member is in contact with the sheet; and due to a difference between the movement speed and the conveyance speed, as the push member pushes the sheet, a portion of the sheet located further downstream than the contact position in the sheet conveyance direction is drawn back in a direction opposite to the sheet conveyance direction and the sheet is drawn into the nip of the second rotating body pair first at the target fold position.