An image forming apparatus which enables double-sided image formation using a sheet feeding unit, and is capable of discharging a sheet, whose sheet length is undetermined by the time the sheet reaches a branch area in conveyance for double-sided image formation, without guiding it to a sheet inverting unit. A conveying flapper switches a destination for a sheet to a first conveying path leading to a sheet inverting unit or a second conveying path leading to a sheet discharge tray. A length of a sheet in a conveying direction is detected while the sheet is being conveyed. When a size of the sheet is undetermined, and the length of the sheet is not detected by the time the sheet reaches the conveying flapper, the conveying flapper is controlled so as to convey the sheet with an image formed on a first side thereof to the second conveying path.