Patent ID: 8218982

Claim:
A developing apparatus having stirring members for stirring a developer-tank-contained developer containing toner and carrier inside a developer tank while conveying said developer and a developer holder disposed adjacent to said stirring members to supply said stirred developer-tank-contained developer to an electrostatic latent image holder, comprising: a developer replenishing tank for replenishing said toner and said carrier to said developer tank, a magnetic-type 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 said developer-tank-contained developer outside said developer tank when the amount of said developer-tank-contained developer inside said developer tank exceeds a predetermined amount, a conveying state switching device for temporarily blocking the flow of said developer-tank-contained developer in said developer tank and for temporarily switching a circulating state of said developer-tank-contained developer to a staying state thereof, and a controller for controlling replenishment operation for replenishing said toner and said carrier for replenishment from said developer replenishing tank to said developer tank when the toner concentration detected using said toner concentration detecting sensor is lower than a predetermined reference toner concentration, wherein said controller calculates the toner concentration on the basis of the output value output from said toner concentration detecting sensor when the staying state is achieved and estimates the amount of said developer inside said developer tank on the basis of the difference between the circulating state output value and the staying state output value, output from said toner concentration detecting sensor, thereby determining the amounts of said toner and said carrier to be replenished on the basis of the calculated toner concentration and the estimated amount of said developer; wherein said conveying state switching device comprises an opening section disposed orthogonal to the conveying passage of said developer tank and a shielding section installed on said stirring member and formed so as to shield said opening section, and the circulating state is generated when said opening section is not overlapped with said shielding section, and the staying state is generated when said opening section is overlapped with said shielding section, whereby the conveying state of said developer-tank-contained developer is switched to the circulating state or the staying state.