A developing device has stirring members for conveying and stirring a developer-tank-contained developer and a developer holder, and has a developer replenishing tank for replenishing a replenishment developer to a developer tank; a toner concentration detecting sensor; a discharging mechanism for discharging an excessive amount of the developer-tank-contained developer outside the developer tank when the amount of the developer-tank-contained developer inside the developer tank exceeds a predetermined amount; and a control unit for replenishing the developer-tank-contained developer when it is detected that the toner concentration inside the developer tank is lower than a first reference value and for forcibly consuming the toner inside the developer tank and replenishing the replenishment developer when it is detected that the toner concentration inside the developer tank is higher than a second reference value that is higher than the first reference value.