A developing including 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 control unit having a memory for storing the current toner concentration inside the developer tank, the reference toner concentration inside the developer tank, a corrected reference toner concentration calculated according to the formula of (the current toner concentration —a correction value), a predetermined replenishment amount to the developer tank and an allowable value for the current toner concentration with respect to the reference toner concentration, and a computer for calculating the corrected reference toner concentration and comparing the current toner concentration with the reference toner concentration and the corrected reference toner concentration.