To obviate a paper jam at the time of conveying a sheet in and out of a switch back conveyor passage.

A switchback unit conveys a sheet taken out of a first conveyor passage 13 into a switchback passage 18a from the conveying tip side, while it moves three reversible rollers 22a, 22b and 22c shiftablly installed to convey the sheet taken into the switch back passage onto a second conveyor passage 24 from the conveying rear end side, and one of them to the opposed position of a sheet conveying outlet of the first conveyor passage. After the sheet conveying in from this sheet conveying ourlet, the reversible roller 22a is moved to the opposed position of a sheet conveying inlet of the second conveyor route 24, while the next revserible roller 22b comprises a sheet conveyor part driving means moving it to the opposed position of the sheet conveying outlet of the first conveyor passage 14 in combination.