Patent ID: 7995935

Claim:
A developer device, comprising: a developer portion which, while agitating, supplies a two-component developer of toner and carrier in a holding chamber to a photosensitive drum to develop an electrostatic latent image formed on the photosensitive drum, and form a toner image; a cartridge which houses toner; a measurement portion which measures the toner concentration in the holding chamber, converts a measurement result into a voltage value and outputs the voltage value; a replenishment portion which replenishes the holding chamber with toner from the cartridge; a replenishment control portion that controls a number of rotations of a toner replenishment motor of the replenishment portion to replenish the holding chamber with toner of a replenishment amount from the cartridge according to the number of rotations; and a calculation portion which calculates a differential sensitivity of the measurement portion obtained by dividing a voltage difference between a first voltage value measured by the measurement portion when a toner concentration of the holding chamber is set to a predetermined first toner concentration by the toner replenishment by the replenishment control portion, and a second voltage value measured by the measurement portion when the toner concentration of the holding chamber is set to a predetermined second toner concentration larger than the first toner concentration by the toner replenishment by the toner replenishment control portion, with a difference between the first toner concentration and the second toner concentration, wherein the replenishment control portion calculates, as a toner concentration at a toner concentration measurement time, a value obtained by subtracting, from the first toner concentration, a value obtained by dividing a difference between a voltage value to be outputted from the measurement portion at the toner concentration measurement time, and the first voltage value, with the calculated differential sensitivity, and which controls replenishment of the holding chamber with toner by the replenishment portion according to the calculated toner concentration.