A sheet processing device performs saddle stitching binding a bundle of stacked paper sheets, and includes a stacker section temporarily stacking conveyed paper sheets substantially vertically; a stopper regulating the paper sheets stacked in the stacker section; a first binding section provided in the stacker and saddle-stitching a paper sheet bundle regulated by the stopper at a binding position around a center of the paper sheet bundle in a sheet conveying direction; a folding section folding in half the paper sheet bundle regulated by the stopper; and a conveying section conveying the paper sheet bundle that has been folded in half by the folding section with a back side of the folded paper sheet bundle as a leading end in the conveying direction. The conveying section includes a second binding section binding the folded paper sheet bundle at the back side thereof.