PURPOSE: To provide a sheet reversing device wherein the size of a main body of a printer is made smaller and the cost thereof is reduced, and the delivery state of sheet, i.e., face-down/face-up state, in one-side/two-side processing can be freely selected according to the requirements of users.

CONSTITUTION: A sheet reversing devide 1 of the invention is provided detachably on a printer 50, and the device 1 introduces sheets printed at a printing unit 59 of the printer 50 to a reversing stacker 40, to which sheets are delivered. And there are provided a guide passage 4 which guides the sheets, in a reversed state, to the stacker 40 so that the sheets are delivered thereto, according to the state of the sheets delivered to a stacker 74 of the printer 50, and a switch-back mechanism 5 and a reversing passage 6 for transferring the sheets in a reversed state to a sheet manual-feed port 60 of the printer 50 by means of the guide means 4 so that two-side printing of the sheets can be performed at the printeng unit 59.