First and second buffer roller pairs are driven such that a sheet conveyed sequentially by a conveying roller pair is overlapped with a standby sheet by shifting in a sheet conveying direction and such that the overlapped sheets standby at a branch path. A shift length in overlapping the standby sheet with the conveyed sheet is set such that the more the number of times of drawal into the branch path of the sheet, the shorter the shift length becomes. Thereby, the shift length between the respective sheets turns out to be a predetermined shift length in overlapping the standby sheets with a final conveyed sheet and conveying the sheets to an intermediate processing tray.