A sheet binding apparatus includes a support for supporting a set of sheets; a puncher for punching the sets of sheets on the support; binder for binding the sheets by penetrating a ring member through punched holes of the sheets punched by the puncher; a detector for detecting a size of the ring member supported on the binder; a changing device for changing a degree of opening of the ring member in accordance with a size of the ring member; a conveyer for conveying the set of sheets punched by the puncher to the ring member supported on the binder; a second support for supporting the binder for movement between an automatic mode position wherein the binder receives the set of sheets conveyed by the conveyer and a manual mode position wherein the set of sheets can be received without use of the conveyer; a second detector for detecting the manual mode position of the binder and for prohibiting conveyance of the sheets by the conveyer; ring controller for controlling the degree of opening of the ring member during the manual mode; wherein the ring controller opens the ring member to the degree of opening predetermined in accordance with the size of the ring member.