A high voltage power supply includes a developer carrier, an image carrier, a developing control unit, a load detection unit, a variation detection unit, and a voltage correction unit. The developing control unit applies a developing bias voltage to the developer carrier and thereby supplies the developer to the image carrier and develops an electrostatic latent image. The variation detection unit detects the variation in capacitance between the developer carrier and the image carrier detected by the load detection unit. The voltage correction unit corrects the developing bias voltage by decreasing the developing bias voltage as the variation increases, and increasing the developing bias voltage as the variation decreases.