Patent ID: 7551862

Claim:
A developing device, comprising: a developer bearing member configured to bear thereon a developer including a toner and a magnetic carrier to develop an electrostatic image on an image bearing member with the developer; a developer container configured to contain the developer therein and feed the developer to the developer bearing member; a toner concentration sensor configured to detect a concentration of the toner in the developer in the developer container and output a signal depending on the detected toner concentration; and a characteristic information storage device provided on a casing of the developing device and configured to store a characteristic of the toner concentration sensor including a blank reference voltage and an initial developer reference voltage prior to the developing device being attached to an image forming apparatus, wherein the characteristic information storage device is separated from the toner concentration sensor and a corrected value of the blank reference voltage and a corrected value of the initial developer reference voltage are compared with the signal output by the toner concentration sensor to determine whether a new developer is present in the developer container.