1. An image forming apparatus comprising: an image forming unit including a latent image carrier configured to carry a latent image on a moving surface thereof, a charging device configured to charge the surface of the latent image carrier, a latent image writing device configured to write the latent image on the charged surface of the latent image carrier, and a development device configured to develop the latent image carried on the surface of the latent image carrier; a cumulative value calculation device configured to calculate, for each of a plurality of regions into which the surface of the latent image carrier is divided in a direction perpendicular to a direction of rotation of the latent image carrier, a cumulative value of the area of the latent image formed in the region; a surface potential detector configured to detect the electric potential of the surface of the latent image in one of the plurality of regions as a detection region; and a determination device configured to: determine the degree of deterioration of the latent image carrier on the basis of readings from the surface potential detector; determine the degree of deterioration of the detection region on the basis of the readings; and determine the degree of deterioration of a region other than the detection region on the basis of the readings, the cumulative value for the detection region, and the cumulative value for the region other than the detection region.