A power supply applies a first charged voltage to a charging section disposed near an image carrier during a predetermined process and applies second charged voltages sequentially to the charging section in obtaining a predetermined peak-to-peak voltage value for an alternating-current voltage of the first charged voltage. Each of the second charged voltages includes an alternating-current voltage having a peak-to-peak voltage value varying between a forward discharge range and a back discharge range. A current sensing section senses the alternating-current running through the charging section upon application of each of the second charged voltages. A control section derives a first approximation function and a second approximation function on the basis of the alternating-current values sensed by the current sensing section and sets a peak-to-peak voltage value as the predetermined peak-to-peak voltage through a difference function representing a difference value between the first approximation function and the second approximation function.