1. An image forming apparatus comprising; an image carrier configured to support an electrostatic latent image, a charger configured to charge the image carrier to an electrical potential according to an applied voltage, an exposure unit configured to form an electrostatic latent image by exposing the image carrier, a developing unit configured to develop the electrostatic latent image by adhering a developer to the image carrier, a detection unit configured to detect the density of a developer adherent to the image carrier, and a controller configured to determine a deteriorated state of the image carrier, by charging the image carrier and then conducting a development with the developing unit, in a condition where a potential difference between the charger and the developing unit is set to be smaller than that at the time of image formation, before detecting a density of a developer on a portion on the image carrier with the detection unit, so that a detection result to be used for its decision of the deteriorated state of the image carrier is obtained, wherein the portion of the image carrier is a portion which is charged and thereafter subjected to the development by use of the developing unit without being subjected to exposure by the exposure unit in the condition where a potential difference between the charger and the developing unit is set to be smaller than that at the time of image formation.