PURPOSE: To provide an image forming device capable of maintaining a stable image.

CONSTITUTION: This device includes an electrifying device 42 applying specified surface potential to a photoreceptor 40 and a surface potential sensor 40a measuring the potential electrified by the device 42 and attenuated with light from an exposure lamp 22. The characteristic of the photoreceptor 40 is decided based on the surface potential attenuated by the lamp 22. In the case the characteristic of the photoreceptor 40 is decided as a noncommendable photoreceptor, an image forming condition is changed by a CPU 70 and respective voltage control circuits 80a, 80b, 82 and 84. Therefore, in the case the non-commendable photoreceptor is utilized, the formed image, that is, the output image is the image formed by the image condition different from the optimum image condition.