An electrostatic latent image is formed on an image carrier. A first roller is opposed to the image carrier with a gap in between. A second roller is in contact with the first roller such that toner of a one-component type is supplied onto the image carrier by way of the first roller to develop the electrostatic latent image as a visible toner image. A single power source supplies a bias voltage in which an AC voltage is superposed on a DC bias voltage. The bias voltage is supplied to the first roller through a first path and to the second roller through a second path. A resistor is provided on the first path between the first roller and a branching point of the first path and the second path.