Patent ID: 7643765

Claim:
An image forming apparatus that charges a surface of a photosensitive member to a predetermined potential using a charging roller, comprising: a bias applying portion that applies a charging bias to the charging roller; a current detecting portion that detects a charging current when the charging bias is applied; a bias correcting portion that carries out correction of the charging bias; a target information storing portion that stores a target charging current value that is taken as a target, that is a charging current value when the surface of the photosensitive member is charged to a required surface potential; and a usage amount information acquiring portion that acquires usage amount information relating to a usage amount of the charging roller; wherein, the bias correcting portion performs a first bias correction operation and a second bias correction operation, in which, the first bias correction operation is an operation that compares a charging current value that is detected by the current detecting portion when a predetermined charging bias is applied by the bias applying portion with a target charging current value that is stored in the target information storing portion, and determines a new charging bias by correcting the predetermined charging bias on the basis of the comparison result, the second bias correction operation is an operation that determines whether a usage amount of the charging roller is a predetermined first usage amount or a second usage amount that is greater than the first usage amount on the basis of usage amount information that is acquired by the usage amount information acquiring portion, and when the usage amount is determined to be the second usage amount, changes the target charging current value in accordance with the usage amount of the charging roller and obtains a new charging bias by correcting the corrected charging bias that is obtained as a result of the first bias correction operation on the basis of the target charging current value that is changed, and the bias correcting portion determines a new charging bias in the second bias correction operation by adding to the corrected charging bias a bias correction value that is calculated using the following formula (a): (Idcα(T)−Idc(m))*k (a) wherein, Idcα(T) represents the target charging current value that is changed, Idc(m) represents a charging current value that is detected by the current detecting portion when the corrected charging bias is applied by the bias applying portion, “k” is a correction coefficient, and the symbol “*” represents multiplication.