A developing device includes a developer carrier opposed to an image carrier for carrying an electrostatic latent image, and a rotatable developer transferring screw for transferring developer to the developer carrier while agitating; the developer carrier includes a magnet roller and a developing sleeve including a rotatable non-electromagnetic cylinder body disposed coaxially with an axis of the magnet roller to contain the magnet roller, and an axis diameter of a center portion of the developer transferring screw is set larger than an axis diameter of each of both end portions of the developer transferring screw.