An image forming apparatus for forming an image on a sheet includes an image bearing drum; a charging member for charging the drum by applying a voltage while contacting the drum; an AC voltage generating unit for generating and applying an AC voltage to the charging member; a state detector for detecting a state of the apparatus; a discharge current detector for detecting a discharge current between the charging member and the drum; a discharge current control unit for controlling the AC voltage generating unit on the basis of a result of detection of the discharge current detector; and a discriminating unit for discriminating whether to calibrate the discharge current detector before formation of the image, in accordance with a result of detection of the state detector.