An image forming apparatus includes a main casing including an image forming unit and a discharge tray, and an optional device configured to be removably attached to the main casing. The main casing includes a conveying path through which a sheet is conveyed from the image forming unit, a discharge path through which the sheet is conveyed toward one of the discharge tray and the optional device, the discharge path diverging from the conveying path, and a return path including a first path through which the sheet is conveyed temporarily out of the main casing via an opening formed in the main casing, and a second path through which the sheet is conveyed toward the image forming unit. The first path diverges from the conveying path. The second path diverges from the first path. The optional device is disposed closer to the discharge tray than the opening.