Patent ID: 7840146

Claim:
A developing device having stirring members for stirring a developer-tank-contained developer containing toner and carrier inside a developer tank while conveying the developer and a developer holder disposed adjacent to said stirring members to supply the stirred developer-tank-contained developer to an electrostatic latent image holder, comprising: a developer replenishing tank for replenishing a replenishment developer containing the toner and the carrier to said developer tank, a toner concentration detecting sensor for detecting the toner concentration inside said developer tank, a discharging mechanism provided in said developer tank to discharge an excessive amount of the developer-tank-contained developer outside said developer tank when the amount of the developer-tank-contained developer inside said developer tank exceeds a predetermined amount, and control unit for replenishing the replenishment developer when it is detected that the toner concentration inside said developer tank is lower than a first reference value and for forcibly consuming the toner inside said developer tank and replenishing the replenishment developer when it is detected that the toner concentration inside said developer tank is higher than a second reference value that is higher than said first reference value.