An image forming apparatus includes an image carrier, a developer carrier, a bias applying section, a leakage detecting section, and a bias controlling section. The developer carrier carries developer in the form of a layer of magnetized filaments to supply toner to the image carrier. The bias applying section applies to the developer carrier a development bias including a direct current voltage VmagDC and an alternating current voltage VmagAC superposed on each other. The leakage detecting section causes a discharge between the image carrier and the developer carrier and detects a leakage voltage when the discharge occurs. The leakage detecting section performs the leakage voltage detection in a blank region of the image carrier bearing no electrostatic latent image. The bias controlling section determines an alternating current voltage VmagAC according to a detected leakage voltage, and a frequency of the alternating current voltage VmagAC according to the voltage VmagAC.