A sheet processing apparatus to perform stapling includes a sheet stacking portion receiving a sheet for a sheet bundle and a positioning member against which an end of the sheet is abutted to position the sheet bundle. The positioning member is movable to adjust a stapling position on the sheet bundle where a staple processing portion performs a stapling process. After the stapling process is performed, a fold processing portion folds the sheet bundle in two. One of two surfaces of the folded sheet bundle slides on a conveyance guide member while a conveyance guide member guides the folded sheet bundle. The stapling process is performed while the stapling position is previously moved, by a predetermined distance, to an opposite area side of the sheet bundle opposite to an area that becomes the one of two surfaces of the folded sheet bundle when the sheet bundle is folded in two.