Developing device and image forming apparatus

An image forming apparatus having a rotatable electrostatic latent image holder for holding electrostatic latent images on the circumferential face thereof, 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 the stirred developer-tank-contained developer to said electrostatic latent image holder, comprising:

• a developer replenishing tank for replenishing the toner and the carrier to said developer tank,
• a toner concentration detecting sensor for detecting the current 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,
• control unit for carrying out control to replenish the toner and the carrier from said developer replenishing tank to said developer tank when said current toner concentration detected using the toner concentration detecting sensor is lower than a predetermined reference toner concentration, wherein
• when a toner concentration higher than said reference toner concentration by a predetermined value or more is detected as said current toner concentration, said control unit corrects said reference toner concentration to a corrected reference toner concentration higher than said reference toner concentration and changes said corrected reference toner concentration so that said corrected reference toner concentration gradually lowers to the value of said reference toner concentration.

A developing method applied to a developing device 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 the stirred developer-tank-contained developer to an electrostatic latent image holder, and a developer replenishing tank for replenishing the toner and the carrier to a developer tank, comprising the steps of:

• detecting the current toner concentration inside said developer tank,
• replenishing the toner and the carrier from said developer replenishing tank to said developer tank when said current toner concentration detected using said toner concentration detecting sensor is lower than a predetermined reference toner concentration,
• discharging an excessive amount of the developer-tank-contained developer outside said developer tank using said discharging mechanism disposed in said developer tank when the amount of the developer-tank-contained developer inside said developer tank exceeds a predetermined amount,
• correcting said reference toner concentration to a corrected reference toner concentration higher than said reference toner concentration when a toner concentration higher than said reference toner concentration by a predetermined value or more is detected as said current toner concentration, and
• changing said corrected reference toner concentration so that said corrected reference toner concentration gradually lowers to the value of said reference toner concentration.