PURPOSE: To provide a developer concentration controller whose circuit constitution is simple and where the temperature characteristic of a light source is corrected by one photodetector.

CONSTITUTION: A developing cylinder 101 is rotated clockwise, and two- component developer consisting of the mixture of toner and carrier sticking to the outer peripheral surface of the cylinder 101 is irradiated with infrared light having a reference light quantity from an LED 102, then a reflected light beam which is reflected by the toner of the developer is received by a photodiode 103. By rotating the cylinder 101 counterclockwise, the developer is removed from the outer peripheral surface of the cylinder 101, which is directly irradiated with the infrared light from the LED 102, and the reflected light beam therefrom is receiving by the photodiode 103. Output signals from the photodiode 103 in accordance with two reflected light beams received are respectively amplified, converted into a digital signal by an A/D 302, and inputted in a CPU 303, thereby controlling the supply amount of the toner so that the ratio of both signal values mat be a fixed value.