A developing device includes: a developing unit; a toner cartridge; a shutter drive member; and a second gear. The toner cartridge is mountable on or detachable from the developing unit and includes a casing defining an internal space for accommodating a developing agent and formed with a communication hole for supplying the developing agent to the developing unit, and a shutter movable between an open position in which the communication hole is open and a closed position in which the communication hole is closed. The shutter drive member includes a first gear that extends in the moving direction of the shutter. The shutter drive member is engageable with the shutter to be movable together with the shutter when the toner cartridge is mounted on the developing unit. The second gear is meshingly engageable with the first gear to transmit a drive force to the first gear. The first gear is disengaged from the second gear when the shutter is in at least one of the open position and the closed position.