An image forming apparatus includes a printer body having a developing portion and a fixing portion therein, a plurality of sheet feeding rollers provided along a sheet discharge path successively and driven to discharge a sheet passing through the fixing portion to an upper portion of the printer body, and a rear cover provided on a side of the printer body to open and close, and having guide ribs to guide the sheet passed through the fixing portion. The image forming apparatus also includes an idle roller provided movably on the rear cover, to contact and rotate with one of the sheet feeding rollers to feed the sheet, and a supplementary guide member provided rotatably on the rear cover and positioned below the idle roller. The supplementary guide member changes a position thereof when the rear cover is open so that the sheet being discharged directly to the guide ribs is prevented from running into the idle roller.