In a developing device (2) for an image forming apparatus and of the type using a developer consisting of toner and magnetic carrier, a toner hopper (8) has an opening for replenishing toner stored therein. The developer is regulated by a doctor blade (6) to form a thin layer on a developing sleeve (4). The developer scraped off by the doctor blade is introduced into a developer storing chamber (10) and caused to move toward the opening (8a) of the hopper due to its own internal pressure and gravity. The developer taken in toner from the hopper is returned toward the doctor blade along the surface of the sleeve. The developer on the sleeve and regulated by the doctor blade is conveyed to a developing position where the sleeve faces an image carrier (1). The toner contained in the developer is a magnetic toner. When the toner concentration of the developer has reached an upper limit, a space or gap exists in the developer storing chamber.