An image forming apparatus that may securely mix a toner and a carrier in a minimum space when high-speed printing is performed. The image forming apparatus including: a photosensitive drum; a developing roller; and a developing unit, wherein the developing unit includes: a receiving or transferring unit for receiving or transferring a two-component developing agent in an axial direction of a developing roller; and a preparatory agitation unit disposed on an end part of the receiving or transferring unit and including an agitation screw that has a large diameter and for mixing the newly-supplied toner and the carrier.