A duplex-printable image forming apparatus which is capable of increasing a speed of duplex printing. The image forming apparatus includes: a developing unit which forms a visible image, which is formed of a developer, on a printing medium; a fixing unit which fixes the visible image formed on the printing medium onto the printing medium; a discharging roller unit which can be regularly/reversely rotated such that the printing medium passed through the fixing unit is conveyed to the developing unit along a duplex printing path or is discharged out of the apparatus; and an intermediate conveying unit which is interposed between the fixing unit and the discharging roller unit and selectively holds the printing medium.