Patent ID: 7684719

Claim:
An image forming apparatus comprising: a photosensitive member; a rotatable charging member configured to electrically charge said photosensitive member; a charging bias applying device configured to apply a superimposed charging bias, comprising a DC voltage and an AC voltage which causes an electrical discharge in a gap between said photosensitive member and said rotatable charging member, to said charging member during an image formation; a toner image forming device configured to form a toner image on said photosensitive member charged by said rotatable charging member; an executing device configured to execute a test mode in which a superimposed test bias, comprising a DC voltage which is not zero and an AC voltage which causes the electrical discharge in the gap, is applied to said rotatable charging member by said charging bias applying device; a detector configured to detect an electrical discharge current from said rotatable charging member to said photosensitive member when the superimposed test bias is applied to said rotatable charging member in the test mode; and a correcting device configured to correct a peak-to-peak voltage of the AC voltage of the superimposed charging bias based on a detection result of said detector, wherein said charging bias applying device applies a plurality of the superimposed test biases of which DC voltages are different from each other in the test mode, said detector detects the electrical discharge current when the superimposed test biases are applied to said rotatable charging member in the test mode, and said correcting device corrects the peak-to-peak voltage of the AC voltage of the superimposed charging bias based on the detection result of said detector.