A developing device for developing a latent image formed on an image carrier of the present invention is generally made up of a developing section and a developer storing section. The developing section includes a developer carrier for conveying a one-component type developer deposited thereon, a feeding member for feeding the developer to the developer carrier, and a conveying member for conveying the developer toward the feeding member while agitating it. The developer storing section replenishes a developer stored therein to the developing section in a direction including at least a horizontal direction component. The developing section and developer storing section are formed with a plurality of openings in the lengthwise direction such that the replenishment of developer from the developer storing section to the developing section and the return of the developer from the latter to the former are executed via the openings.