An image forming apparatus includes an image carrier and the following elements. A charging unit charges the image carrier. An exposure unit exposes the charged image carrier to light and forms an electrostatic latent image. A developing unit generates a developing electric field and develops the electrostatic latent image. A transfer unit transfers the developed image. A controller outputs an AC setting signal. The developing unit includes a bias power supply source having the following elements. An output transformer includes a primary winding and a secondary winding. A switching circuit supplies a current to the primary winding. A current control circuit includes first impedance and second impedance. The first impedance is set when the AC voltage has a first frequency and the second impedance is set when the AC voltage has a second frequency, thereby controlling a current flowing between the primary winding and the switching circuit.