US Patent 7925175 Developing device and image forming apparatus including the same

A developing device includes a developer container storing a developer, a developer supplying member supplying a toner stored in the developer container to an image bearing member, a voltage applying unit capable of applying a bias voltage to the developer supplying member, an image density detecting unit detecting a density of toner formed on the image bearing member, and a control unit correcting a toner concentration in the developer on the basis of a result of image density detection of a reference toner image formed on the image bearing member. The control unit is capable of executing a first toner concentration correction between printing operations and executing a second toner concentration correction between predetermined points during continuous printing.