Patent ID: 7590365

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; and 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; 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; and the second bias correction operation is an operation that determines whether a corrected charging bias that is obtained as a result of the first bias correction operation is at a predetermined first level or at a second level that is higher than the first level, and when the corrected charging bias is determined to be at the second level, changes the target charging current value in accordance with the corrected charging bias and obtains a new charging bias by correcting the corrected charging bias on the basis of the target charging current value that is changed, wherein 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.