A developing device incorporated into a copying machine of the present invention has a developer/toner supplying section in which a developing tank and a toner cartridge are independently and detachably formed. A toner density sensor for controlling toner density of developer is provided in the developing device. The toner density sensor is used to detect a developer drawing defect and maintain a predetermined value of toner density during the copying operation when developer is supplied from the developer/toner supply section.