A developing cartridge comprising: a casing configured to accommodate a developer therein; a developing roller extending in a first direction; a developing-roller gear mounted to the developing roller and rotatable with the developing roller, the developing-roller gear being positioned at an outer surface of the casing; a coupling rotatable about a first axis extending in the first direction and positioned at the outer surface, the coupling including a coupling gear meshing with the developing-roller gear, the coupling gear being rotatable with the coupling; a first idle gear meshing with the coupling gear and rotatable about a second axis extending in the first direction, the first idle gear being positioned at the outer surface; a second idle gear rotatable with the first idle gear about the second axis, the second idle gear being positioned at the outer surface and spaced apart farther from the outer surface than the first idle gear from the outer surface, wherein a diameter of the second idle gear is smaller than a diameter of the first idle gear; an agitator extending in the first direction; a first agitator gear mounted to the agitator and rotatable with the agitator about a third axis extending in the first direction, the first agitator gear being positioned at the outer surface and meshing with the second idle gear; and a protrusion extending in the first direction, the protrusion being positioned between the first axis and the third axis in a second direction connecting the first axis and the third axis and positioned at the outer surface, the protrusion being positioned outside an addendum circle of the developing-roller gear, outside an addendum circle of the coupling gear, outside an addendum circle of the first idle gear and outside an addendum circle of the second idle gear, characterized in that the first agitator gear is spaced apart from the protrusion in the first direction, wherein the protrusion is configured to receive a pressing force from a drum cartridge in response to mounting the developing cartridge to the drum cartridge and wherein an edge of the first agitator gear in the first direction is spaced apart farther from the outer surface than the protrusion from the outer surface not to prevent the gear part from rotating even though the protrusion is positioned within an addendum circle of the gear part in the second direction. a casing configured to accommodate a developer therein; a developing roller extending in a first direction; a developing-roller gear mounted to the developing roller and rotatable with the developing roller, the developing-roller gear being positioned at an outer surface of the casing; a coupling rotatable about a first axis extending in the first direction and positioned at the outer surface, the coupling including a coupling gear meshing with the developing-roller gear, the coupling gear being rotatable with the coupling; a first idle gear meshing with the coupling gear and rotatable about a second axis extending in the first direction, the first idle gear being positioned at the outer surface; a second idle gear rotatable with the first idle gear about the second axis, the second idle gear being positioned at the outer surface and spaced apart farther from the outer surface than the first idle gear from the outer surface, wherein a diameter of the second idle gear is smaller than a diameter of the first idle gear; an agitator extending in the first direction; a first agitator gear mounted to the agitator and rotatable with the agitator about a third axis extending in the first direction, the first agitator gear being positioned at the outer surface and meshing with the second idle gear; and a protrusion extending in the first direction, the protrusion being positioned between the first axis and the third axis in a second direction connecting the first axis and the third axis and positioned at the outer surface, the protrusion being positioned outside an addendum circle of the developing-roller gear, outside an addendum circle of the coupling gear, outside an addendum circle of the first idle gear and outside an addendum circle of the second idle gear, characterized in that the first agitator gear is spaced apart from the protrusion in the first direction, wherein the protrusion is configured to receive a pressing force from a drum cartridge in response to mounting the developing cartridge to the drum cartridge and wherein an edge of the first agitator gear in the first direction is spaced apart farther from the outer surface than the protrusion from the outer surface not to prevent the gear part from rotating even though the protrusion is positioned within an addendum circle of the gear part in the second direction.