PURPOSE: To obtain a device which is not affected by the dirt of a photodetector or a light emitting element, by intercepting the incidence of a measuring beam to the photodetector through a shutter or the like when image density is measured and compensating the image density measured value on a basis of the photodetection output at this time.

CONSTITUTION: An image density detecting unit 5 is provided with a shutter 17, and the light is not incident to a photodetector 7 when the shutter 17 is closed. The output from the photodetector 7 is inputted to a central controlling circuit 19 through an input circuit 18, and the data processing is performed in the controlling circuit 19 on a basis of the output from the photodetector 7 and other signals, and the result controls a laser turn-on/off controlling circuit 21, a developing motor controlling circuit 22, a shutter open/close controlling circuit 23 of the image density detecting unit, and a turn-on/off contolling circuit 24 for a toner hopper motor through an output circuit 20. A motor driving circuit 15 is controlled by the controlling circuit 24 to control the quantity of toner supply from a hopper 2 to a developer 3.