A developing device for an image forming apparatus and of the type using a developer consisting of toner and magnetic particles is disclosed. While a developing sleeve conveys the developer deposited thereon, toner is automatically replenished to the developer from a toner hopper via an opening formed in the hopper due to the movement of the developer. At the same time, the toner introduced into the developer is charged by friction. A developer storing member includes a penthouse portion. A projection protrudes from the surface of the penthouse portion facing the developing sleeve toward the sleeve and extends in the direction perpendicular to the direction in which the sleeve conveys the developer. The developer forms on the sleeve a layer having stable toner content and stable toner charge without resorting to a sophisticated toner content control mechanism.