Patent ID: 8023839

Claim:
A developing apparatus having stirring members for stirring a developer containing toner and carrier inside a developer tank while conveying said developer and having a developer holder disposed adjacent to said stirring members to supply said developer to an electrostatic latent image holder, comprising: a developer replenishing tank containing replenishment toner and replenishment carrier; a toner concentration detecting sensor for detecting a toner concentration inside said developer tank; a developer amount estimating sensor for estimating an amount of said developer inside said developer tank; a discharging mechanism provided in said developer tank to discharge an excessive amount of developer outside said developer tank when said amount of said developer inside said developer tank exceeds a predetermined amount; and a controller for controlling a replenishment operation for transferring a mixture of said replenishment toner and said replenishment carrier from said developer replenishing tank to said developer tank when said toner concentration detected using said toner concentration detecting sensor is lower than a predetermined reference toner concentration, wherein said controller determines specified amounts of said replenishment toner and said replenishment carrier to be transferred from said developer replenishing tank to said developer tank based on said detected toner concentration and said estimated amount of said developer.