An image forming apparatus has exposure unit for emitting light modified in accordance with image information, the light quantity of said exposure unit being variable, image forming unit for forming image on a photosensitive medium in accordance with light emitted from the exposure unit, detector for detecting the surface state of the photosensitive medium, and controller for optimizing the light quantity of the exposure unit in accordance with the output of the detecting unit. The control unit obtains a potential versus image information variation characteristic of the photosensitive medium based on the output of the detecting unit and sets the light quantity of the exposure unit such that the characteristic approximate a straight line.