A toner-concentration controller includes 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 two-component toner. The sensor unit includes a correction mechanism to correct an output signal of the sensor unit by changing an external-input voltage, 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. 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.