A developing device includes a developing roller that carries a two-component developer from a developer container to an image carrier, a first screw carrying a developer, and a second screw carrying the developer in a direction opposite to a direction in which the first screw carries the developer. The first screw and the second screw circularly convey the two-component developer while stirring the two-component developer to supply the two-component developer to the developing roller. A rotation speed of the second screw and a rotation speed of the developing roller can be drive-controlled independently, and a ratio of a rotation speed between the second screw and the developing roller is variable.