PURPOSE: To prevent toner from failing to be charged electrostatically by comparing the signal from a toner concentration detecting means with at least two high and low comparison levels and varying stepwise the amount of toner supplied each time.

CONSTITUTION: The concentration of the toner in a developing tank is detected by a toner concentration detector 10 and compared with reference values and when the toner concentration decreases below the low-level-side reference value, toner is supplied to the developing tank 3 by a supplementary roller 9. The supplied toner is agitated and mixed with carriers smoothly by an agitator screw and the supply of the toner raises the toner concentration in the developing tank 3 speedily, so that image density begins to recover. When the toner concentration in the developing tank 3 rises above the low-level-side reference value, the supply pitch of the toner is decreased. Consequently, the newly supplied toner becomes easy to mix with carriers and are charged electrostatically and sufficiently to reduce a scatter of toner as much as possible, thereby improving picture quality and preventing staining.