Patent ID: 8634730

Claim:
An image forming apparatus comprising: a toner container storing a toner; a developing unit performing development using a dual-component developer containing the toner and a carrier; a toner supply unit supplying the toner from the toner container to the developing unit; a toner sensor detecting a presence or absence of the toner falling and supplied into the developing unit; and a control unit controlling an operation of image forming, the control unit including a toner concentration controller that controls so that a toner concentration of the dual-component developer will fall within a predetermined range, wherein when detecting that an input gain of the toner sensor falls out of the predetermined range, the toner concentration controller adjusts the input gain of the toner sensor so as to fall within the predetermined range and then control a supplied amount of the toner, and the predetermined range for the input gain of the toner sensor includes a pair of first thresholds as criteria for determining whether supply of the toner should be made by performing feedback control in accordance with the input gain of the toner sensor, and a pair of second thresholds as the criteria for determining whether the image forming apparatus is in a breakdown condition so that the image forming operation should be prohibited.