Patent ID: 7389060

Claim:
An image forming apparatus, comprising: an image carrying member configured to form a reference latent image pattern thereon; a developing unit configured to develop the reference latent image pattern as a reference image pattern with a two-component developer having toner and a carrier; an image density detector configured to detect an image density of the reference image pattern; a toner concentration detector configured to detect a toner concentration in the two-component developer; a toner concentration controller configured to control the toner concentration in the two-component developer based on the image density detected by the image density detector; an output adjustment unit configured to adjust a toner concentration sensing level of the toner concentration detector with a sensor control voltage; and a controller configured to control the toner concentration in the two-component developer by changing a target voltage based on the image density detected by the image density detector, the target voltage being set in advance for regulating the toner concentration in the two-component developer and having a corresponding voltage range for changing the target voltage, wherein the controller is configured to control the toner concentration in the two-component developer in a first toner concentration range having an upper and lower limit by changing the target voltage in the voltage range, and when the toner concentration becomes one of the upper and lower limit of the first toner concentration range, a second toner concentration range, which is broader than the first toner concentration range, is set for the toner concentration range by changing the sensor control voltage so that the toner concentration is controlled in the second toner concentration range.