To provide a sheet aftertreatment device for giving aftertreatment to sheets discharged from a sheet treatment device such as an image forming device including a copying machine and a printer, capable of easily and simply changing position and/or direction of aftertreatment on the sheets.

This aftertreatment device includes a sheet placement tray A on which sheets S discharged from a sheet treatment device is placed and a stapler D by which binding treatment (one example of preset aftertreatment) is given to the sheets S on the sheet placement tray A. Herein, the stapler D is demountable from a sheet aftertreatment device body and changeable in mounting position and/or direction with respect to the sheet aftertreatment device body with the mounting/demounting operation of the stapler D so that the position and/or direction of aftertreatment on the sheets S can be selected.