A developing device includes a developing roller and a developing magnetic pole. The developing roller is disposed to face an image carrier across a predetermined gap. The developing magnetic pole is provided in the developing roller so as to be displaced between first and second magnetic pole positions. When the developing magnetic pole is at the first magnetic pole position, a magnetic field is generated at a first position in the gap. When the developing magnetic pole is at the second magnetic pole position, a magnetic field is generated at a second position in the gap. When a circumferential speed ratio of the developing roller to the image carrier is increased, the developing magnetic pole is displaced toward the second magnetic pole position, and when the circumferential speed ratio is decreased, the developing magnetic pole is displaced toward the first magnetic pole position.