A developing device and a developer cartridge are provided. The developing device includes a first housing that receives a developing roller; a second housing that is disposed adjacent to and detachable from the first housing and stores developer; first and second communication paths that allow an inside of the first housing to communicate with an inside of the second housing and an agitator that is provided within the second housing and conveys the developer from the first communication path toward the second communication path. The developing cartridge includes a housing that stores developer and an agitator including a rotational shaft that is disposed in one direction and a first blade that is provided at a midpoint in the axial direction of the rotational shaft and conveys the developer from the midpoint in the axial direction of the rotational shaft to both end portions of the rotational shaft.