To temporarily hold paper, which carries an image formed on at least its one side and is discharged from a discharge port 27 or 28, inside a conveying passage 25 of an optional device 41 by installing the optional device 41 in an image forming apparatus 1 having the discharge ports 27 and 28.

After forming an image by a printer engine 21, the paper is discharged from the discharge port 28 via a conveying passage 34 to be conveyed into a conveying passage 45 from an entrance 49 of the optional device 41. After finishing the image formation in this way, the paper with a long length can be taken out manually from the optional device 41 when opening a lid 43. Alternatively, by operating an operation panel 7, the paper is conveyed through the conveying passages 45 and 37 to be discharged from a discharge port 35, and can be taken out (arrows (3) (4) (5)).