Image forming system and paper sheet output method

A paper sheet output method including:

• a first transporting unit that transports paper sheets along a first transportation route;
• an image forming unit that forms images on the paper sheets transported by the first transporting unit;
• and a sheet supplying unit that has a second transportation route, and transports a sheet along the second transportation route to supply the sheet to a gap between the paper sheets sequentially transported along the first transportation route, the second transportation route joining the first transportation route at a position located on a downstream of the image forming unit in a paper sheet transporting direction, the paper sheet output method comprising:
• outputting, to a first output unit, a paper sheet located on a downstream of the gap between the paper sheets in the paper sheet transporting direction when the sheet is not supplied from the sheet supplying unit to the gap between the paper sheets;
• and
• outputting, to a second output unit different from the first output unit, a paper sheet located on an upstream of the gap between the paper sheets in the paper sheet transporting direction while causing the image forming unit to form an image on the paper sheet located on the upstream of the image forming unit, when the sheet is not supplied from the sheet supplying unit to the gap between the paper sheets.