PURPOSE: To control the concn. of a toner accurately at a constant value in the stage of controlling the rate of toner replenishment according to the image density of the toner by correcting the rate of toner replenishment in accordance with the quantity of the surface potential of a recording body.

CONSTITUTION: A reference image is formed on a photosensitive drum 1, and after the potential of the latent image of said reference image is measured with a surface potential sensor 14, the image is developed with a developer. The concn. of the developed reference image is detected with a detecting means 5. The potential Vl of the latent image and the reflected light output Bm from a detecting means for the concn. of the reference image are inputted respectively to a central control circuit 13. The density of the developed image varies with the surface potential of the latent image and when it is developed by using the same developer, there is the relation: the output B of the reflected light B= aVl+b(a, b are constants). Therefore, the output is calculated according to B= a(Vl0-Vl)+Bm(Vl0 is the standard set value of the surface potential) from the above-mentioned measured value, and according to said value, the toner is replenished 2.