An image-forming device includes an image acquiring unit, an image forming unit, a determining unit, and a controller. The image acquiring unit acquires image data for one page. The image forming unit forms an image based on the image data. The image forming unit includes a developing roller, a supply roller, and a bias applying unit. The developing roller carries a developer. The supply roller supplies the developer to the developing roller. The bias applying unit applies a first voltage between the developing roller and the supply roller in a first mode, and applies a second voltage between the developing roller and the supply roller in a second mode. The determining unit determines whether the image data includes a part having density greater than or equal to a prescribed density. The controller controls the bias applying unit to apply the first voltage when the determining unit determines that the image data does not include the part having density greater than or equal to a prescribed density, and to apply the second voltage when the determining unit determines that the image data includes the part having density greater than or equal to a prescribed density.