1. An image forming apparatus comprising: a rotatable latent image bearer to bear a latent image on its surface; a charging device to electrically charge the surface of the latent image bearer; a latent image writing device to write the latent image on the electrically charge surface of the latent image bearer after a charging process of the charging device; a developing device to develop the latent image borne on the surface of the latent image bearer; a surface potential detector to detect a potential of the surface of the latent image bearer; a deterioration determining device to determine a deterioration degree of the latent image bearer based on a potential detected by the surface potential detector; a counter to count and aggregate the number of pixels output after the latent image bearer is newly used to each of virtually divided regions of the latent image bearer, the virtually divided regions of the latent image bearer defined by virtually dividing the surface of the latent image bearer into multiple regions in a direction perpendicular to a rotating direction of the latent image bearer; and a time value determining device to determine a time value in accordance with a time at which a pixel is output to the virtually divided region, wherein the surface potential detector detects a potential of a detecting target region of the virtually divided region, wherein the deterioration determining device calculates a relative value based on the aggregated number of pixels and the time value per virtually divided region, the deterioration determining device determining a deterioration degree of the detecting target region of the virtually divided region based on the potential detected by the surface potential detector, the deterioration determining device further determining the deterioration degrees of non-detecting target respective regions of the virtually divided regions other than the detecting target region based on the relative values of the non-detecting-target respective regions, the relative value of the detecting target region, and the deterioration degree of the detecting target region.