PURPOSE: To control image density rapidly and accurately, by partially forming surface potential of almost 0 V little affected by charging and exposure characteristics, and attaching toner there.

CONSTITUTION: When a density detecting circuit 26 is turned on with a timing generating circuit 25 and voltage is applied from a power supply 29 to a reverse charger 30 by changeover of power, since a shielding plate 31 is provided before a corona wire 30b, the surface of a photoreceptor 1 is charged separately to potential reverse to normal charging polarity on one part, and to almost 0 V potetial on the part shielded with the plate 31. On the other hand, the contact point of a developing roll 17 is changed over to 32b to connect a bias voltage source 34 equal in polarity to the reversely charged part on the photoreceptor 1, and the toner is attached only to the 0 V part on the photoreceptor 1. An amt. of toner attached is detected with a detector comprising a light emitting element 6 and a photodiode 7, and when it is smaller than the reference value, toner concn. in a developer is raised and when it is not raised in spite of these measures, other factors of development characteristics are checked and restored to good condition.