A developing unit includes a developing hopper for storing a dual-component developer. In the developing hopper, an opening portion is formed at the position opposing the outer peripheral surface of a photoreceptor drum. A developing roller for supplying the developer to the photoreceptor drum to develop an electrostatic latent image is provided inside the developing hopper. The developing roller has a multi-pole magnetic member having a multiple magnetic poles and a non-magnetic sleeve. The multi-pole magnetic member has a multiple number of magnetic poles radially arranged apart from each other. The photoreceptor drum incorporates an opposing magnetic pole formed of a bar magnet having a rectangular section, disposed at a position opposing the magnetic pole across a photosensitive layer. The opposing magnetic pole has a polarity dissimilar to that of the main magnetic pole.