A sheet processing device includes a pair of gripping rollers which can grip and convey a sheet bundle in a booklet-like form, a pressing roller which presses and flattens the folded back portion of the sheet bundle gripped by the gripping rollers, and a control unit which performs rotation control on the pair of gripping rollers. The control unit controls the gripping rollers so as to stop when the folded back portion of the sheet bundle protrudes from the rollers by a predetermined amount. The pressing roller flattens down the protruding folded back portion when the gripping rollers are stopped by the control unit and is in the state of gripping the sheet bundle.