Patent ID: 7720402

Claim:
A toner-concentration controller, comprising: a controller configured to control a toner supply amount in accordance with a detection result of a toner-concentration of two-component toner; and a sensor unit configured to detect the toner-concentration of the two-component toner, the sensor unit including, a correction mechanism configured to correct an output signal of the sensor unit by changing an external-input voltage input to the sensor unit when a process linear velocity changes, and the external-input voltage is changed to match a relationship between the toner-concentration and an output voltage at a normal linear velocity, based on relationship data between an output voltage change of the sensor unit and a toner-concentration of unused developer, to control the toner supply amount when the toner-concentration of the two-component toner deviates a predetermined amount from the toner-concentration of the unused developer, wherein the sensor unit is configured to detect the toner-concentration of the unused developer from unused two-component toner based on a change in the external-input voltage.