An image forming apparatus including an AC high voltage power supply, a DC high voltage power supply, and a high voltage power supply apparatus, which outputs an output voltage at an output end by superposing an output of the AC high voltage power supply and an output of the DC high voltage power supply. The high voltage power supply apparatus has a positive peak detector detecting a positive peak of the voltage at an output end, and a negative peak detector detecting a negative peak of the voltage at an output end. An output voltage of at least one of the AC high voltage power supply and the DC high voltage power supply is controlled on the basis of the detection result of at least one of the positive peak detector and the negative peak detector. An output voltage of the AC high voltage power supply is controlled on the basis of the detection result of the positive peak detector, and an output voltage of the DC high voltage power supply is controlled on the basis of the detection result of the negative peak detector.