A sheet binding apparatus includes a supporting member for supporting a set of sheets; a punch for punching the set of sheets on the supporting member; a binder for supporting a ring member and for binding the sheets by penetrating the ring member through punched holes of the sheets punched by the punch; a detector for detecting a size of the ring member supported on the binder; a device for changing a punching condition by the punch in accordance with an output of the detector; and a conveyer for conveying the set of sheets punched by the punch to the ring member supported on the binder.