<p>Deteriorating developer including toner and carrier in a developer container (16) is replaced by supplying carrier developer including carrier from a developer supply unit (20) and discharging an amount of the deteriorated developer corresponding to the amount of the supplied carrier developer through a discharge opening (24) formed in the developer container (16) by opening a cover. The cover is driven by a solenoid for discharging. With this structure, even when the amount of developer in the developer container is changed and the surface level of the developer is changed, a predetermined amount of deteriorated developer is accurately discharged according to the driving of the cover. It is thus possible to control in a desired manner the replacement ratio of deteriorated developer in the developer container to new developer to be supplied. With this configuration, it is possible to maintain the charge of developer substantially uniform and good image quality of copies. <IMAGE></p>
