Two-dimensional numerical study was carried out on the problem in charging known as the pin-hole leak phenomenon caused by photoreceptor defects in the roller charging system under constant current control with the DC voltage superposed on the AC voltage applied using the finite difference method in the generalized coordinate system. In this simulation, the pin-hole leak phenomenon was numerically reproduced by constructing a roller charging system model with photoreceptor defects and its mechanism was clarified. Further, the effects of the BCR (Biased Charging Roller) parameters to reduce the size of the image defect were quantitatively investigated. Consequently, it was suggested that larger BCR resistivity and higher AC frequency were both effective.