The present invention provides an image forming apparatus which includes a rotatable photosensitive member, a charging device which charges the photosensitive member, an applying unit which applies an alternating current voltage to the charging device, a processing portion which extracts a discharge current component by removing an alternating current component corresponding to the alternating current voltage from a current flowing between the photosensitive member and the charging device, and a control unit which controls a peak-to-peak value of the alternating current voltage based on the discharge current component extracted by the processing portion.