A sheet processing apparatus including a stacking tray that stacks sheets, a conveying member that conveys a sheet to the stacking tray and discharges the 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. When the conveying member conveys the sheet to the stacking tray, a part of the conveying belt that is not wound on the stretch roller contacts the conveying roller by moving the conveying belt as such a nip for conveying the sheet is formed. When the conveying member discharges the sheet bundle from the stacking tray, a part of the conveying belt that is wound on the stretch roller contacts the conveying roller by moving the conveying belt so that a nip for conveying the sheet is formed.