Patent ID: 7558496

Claim:
An image forming device, comprising: an image carrier; a developing device being configured to develop electrostatic latent images formed on said image carrier using developer including carrier and toner, said developing device having a developing roller being configured to supply toner to said image carrier, and a first developer agitation screw being configured to agitate said developer and to transport said developer to said developing roller, said first developer agitation screw having a blade; a toner supply device being configure to supply toner to said developing device; a toner concentration sensor being configure to measure toner concentration within said developing device, said blade being configured to gather developer onto said concentration sensor at a first predetermined period of time; and a toner concentration control unit being configure to control said toner supply device based on the output of said toner concentration sensor, said toner concentration control unit being configured to detect a maximum value of said output of said toner concentration sensor during a second predetermined period of time being longer than said first predetermined period of time, and said toner concentration control unit being configured to control toner concentration using the obtained maximum value as a characteristic value.