A developer cartridge and developing device are provided. The developer cartridge is attachable to and detachable from a developing housing, the developing housing supporting a developer carrier and comprising a drive gear for transmitting a driving force to the developer cartridge. The developer cartridge includes a cartridge housing that is configured to accommodate developer, an agitator that is provided in the cartridge housing and is configured to agitate the developer by the driving force, a transmission gear that is configured to mesh with the agitation gear, the transmission gear meshing with the drive gear when the developer cartridge is installed in the developing housing, and a supporting member that is configured to support the transmission gear movably with respect to the agitation gear while the transmission gear meshes with the agitation gear.