A developing cartridge including: a housing capable of accommodating therein toner; a developing roller rotatable about a first axis extending in an axial direction; a supply roller rotatable about a second axis extending in the axial direction; a coupling rotatable about a third axis extending in the axial direction; a developing roller gear rotatable about the first axis together with the developing roller; a supply roller gear rotatable about the second axis together with the supply roller; a large-diameter gear rotatable about the third axis together with the coupling and meshing with the developing roller gear; a small-diameter gear rotatable about the third axis together with the coupling and having a diameter smaller than a diameter of the large-diameter gear, and an idle gear rotatable about a fourth axis extending in the axial direction and meshing with the supply roller gear and the small-diameter gear.