To prevent mismatching of sheets caused by difference of a position of a sheet stopper of a staple unit in association with a change of a binding pitch when back edges of the sheet are to be matched.

A sheet treating apparatus comprises an initial mode for treating to bind two positions of ends of a sheet bundle matched on a treating tray 400 at a predetermined binding pitch, an a binding pitch changing mode for treating to change the binding pitch within a predetermined range and to bind the two positions of the ends of the sheet bundle matched on the tray 400 at a binding pitch in response to the change. The staple unit 500 has a rear end stopper 401 of the tray 400 and a sheet stopper 513 for specifying the ends of the sheets on the tray 400. The unit 500 treats to bind a first position at the matched position in the initial mode and matches the sheet at the matching position, holds the matched sheet bundle at a bundle discharge roller pair 406, then moves the bundle from the matched position to the first binding position, and first binds at the first position in the binding pitch changing mode.