An image forming apparatus charges the surface of a photosensitive member to a predetermined potential. A current detector detects a charging current when the charging bias is applied. A storage stores a target charging current value when the photosensitive member is charged to a required potential. A usage acquiring portion acquires information relating to usage of the charging roller. A bias corrector performs first and second corrections. The first correction compares a charging current value with a stored target charging current value and determines a new charging bias based on the comparison. The second correction determines whether usage of the charging roller is a predetermined first amount or a greater amount based on acquired usage information, and when the usage is the greater amount, changes the target charging current value in accordance with the usage amount and obtains a new charging bias based on the changed target charging current value.