PURPOSE: To provide an image forming device capable of stabilizing the concentration of a developer.
CONSTITUTION: In the image forming device provided with a developer concentration detector (an LED 1 and a photodiode 2) outputting a detection signal corresponding to the concn. of the developer constituted by mixing carrier grains with toner grains, a developer concn. controller setting a concn. reference value in response to the detection signal outputted from the developer concn. detector, comparing the concn. reference value with a detection signal value and replenishing toner based on the value of the comparison and a developing device 9K carrying the developer to a developing part facing a latent image formed on a photosensitive drum 4K, to develop the latent image, the concn. reference value is set or/and changed based on the history of the developer. Thus, the peculiar phenomenon of the developer concn. controller that the concn. of the developer is stabilized in a rather high level by the history of the developer is suppressed by the setting or/and changing of the concn. reference value and the concn. of the developer converges at a reference concn. and is stabilized.
SAKAMI YUJI