<p>A developing unit (2a) using a two-component developer (24) intended to provide an image forming apparatus capable of forming a high-quality image for a long period of time. A developing unit (2a) using a developer (24) contains toner, carrier and opposite polarity particles having a polarity opposite to the charging polarity of toner includes separation means (22) for separating the toner or opposite polarity particles, and control mechanism (Vb1,Vb2) for controlling opposite polarity particle separation ratio in response to the image area ratio and the number of prints.</p>
