Sheet processing apparatus and image forming system

An image forming system, comprising:

• an image forming unit that forms an image on a sheet;
• a stacking tray that stacks sheets, the sheets including respective images formed thereon;
• a conveying member that conveys the sheet including the image to the stacking tray and discharges a sheet bundle from the stacking tray, wherein the conveying member includes a conveying roller and a conveying belt stretched by a plurality of stretch rollers;
• and
• a sheet processor that performs predetermined processing to the sheet bundle,
• wherein when the conveying member conveys the sheet to the stacking tray, a part of the conveying belt that is not wound on the stretch rollers contacts the conveying roller by moving the conveying belt or the conveying roller so that a belt nip for conveying the sheet is formed, and when the conveying member discharges the sheet bundle from the stacking tray, a part of the conveying belt that is wound on the stretch rollers contacts the conveying roller by moving the conveying belt or the conveying roller so that a roller nip for conveying the sheet is formed, and
• wherein the conveying belt is relatively movable to the conveying roller so that the conveying belt is apart from the conveying roller.