PURPOSE: To provide an image forming device capable of setting the sensitivity of a density sensor adaptive to a use condition in a photosensitive body side, in particular to a new use condition when the photosensitive body is changed.

CONSTITUTION: A device using an electrostatic copying process is provided with a photosensitive body protective sheet 201 having a reference reflection part with a specified reflective rate set at least in a part and this device is capable of correcting the sensitivity of a concn. sensor 41 for controlling a toner concn. by comparing the output of the concn. sensor 41 obtained from the reference reflection part of the photosensitive body protective sheet 201 and that obtained from a ground part in which the ratio of the photosensitive body is equivalent to an image part. Thus, even when a developer whose characteristic changes depending on an environment is used, information necessary for toner concn. control is obtained by means of a ground part potential and a detected output from the concn. sensor irrespective of detection based on visible patterns.