PURPOSE: To control image density stably and exactly, by utilizing the merit of the first detection mode consisting mainly of controlling an amt. of toner to be replenished, and correcting its demerit by the second detection mode.

CONSTITUTION: A reference relfective plate is installed on the outside of the image forming region of an original placing glass, and when an amt. of toner attached is smaller than the reference amt. as a result of a detector comprising a light emitting element 6 and a photodiode 7, the conce. of the toner in a developer is raised, but when deterioration of image density is detected in spite of these measures, the detection mode is changed over from the first to the second mode. The surface of a photosensitive drum 1 is uniformly charged with a charger 2, and uniformly exposed to light from an erasion lamp 4 to redude the surface potential of the drum 1 to almost to 0 V, then developed with a developing roll 17 having reverse bias applied from a power supply 34, and an amt. of toner attached is again detected with said detector comprising the devices 6, 7. When it is judged that it is similar to the reference value, or higher than this, staining of the charger 2, etc. are checked and restored to good conditions, and the mode is returned to the first normal detection mode.