An image forming apparatus includes a photoconductor, a charging member, a first power supply circuit which supplies electric power to the charging member, a detector configured to detect a current value of an alternating current which flows to the charging member, and a controller configured to control an operation of the first power supply circuit. The controller is configured to lower a frequency of alternating-current power supplied to the charging member by the first power supply circuit when the current value detected by the detector in application of a voltage of a prescribed value to the charging member by the first power supply circuit is equal to or smaller than a predetermined value.