A sheet processing apparatus comprising a sheet stacking member for stacking a sheet to be discharged, and an offset mounting member for offsetting a plurality of sheet bundles on sides in a sheet bundle takeout direction and in a direction opposite thereto, and mounting the offset bundles onto the sheet stacking member. The offset mounting member mounts the first sheet bundle on the side in the sheet bundle takeout direction. When there is a sheet bundle on the sheet stacking member, the first sheet bundle is mounted by offsetting the first sheet bundle in a direction opposite to that of the last sheet bundle mounted.