A method of reusing a developing device that can be detachably mounted in a body of an image-forming device includes: preparing a used developing device that includes a developer-carrying member that carries a developer thereon, and a gear mechanism having a plurality of gears that transfer a driving force inputted from the image-forming device to the developer-carrying member; and replacing at least one gear in the gear mechanism with at least one replacement gear that has stronger gear teeth, gear teeth that have a larger module, or gear teeth that have a greater working depth than the at least one original gear.