A developing device includes a developer carrier, a developing case and a toner cartridge. The toner cartridge includes a toner case including a first opening and a first blocking member. The developing case includes a second opening, a second blocking member and an opening/closing member. The opening/closing member is swingable among a first position where the toner cartridge is detached from the developing case and the second opening is closed, a second position where the toner cartridge is attached to the developing case and the first opening and the second opening are communicate with each other, and a third position between the first position and the second position where the toner cartridge is attached to the developing case and the first opening and the second opening are closed. The opening/closing member swings from the first position to the third position when the toner cartridge is attached to the developing case.