A developing device includes a developing housing that supports a developer carrier, and a 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 a driving force, and a transmission gear that is configured to transmit the driving force to the agitator. The developing housing comprises a moving gear that is movable between a meshing position in which the moving gear meshes with the transmission gear and a spaced-apart position in which the moving gear is spaced apart from the meshing position. The moving gear is configured to transmit the driving force from the transmission gear to the agitator.