A toner cartridge and a developing unit are used in an image forming apparatus. The toner cartridge includes a toner chamber, an inner shutter, and an outer shutter. The toner chamber holds toner therein and has an opening through which the toner is discharged into the developing unit. The inner shutter is disposed in the toner chamber and is rotatable relative to the toner chamber to close and open the opening. The outer shutter has an engagement portion, is mounted to an outer surface of the toner chamber, and is movable either to a closing position where said outer shutter closes the opening or to an opening position where the outer shutter opens the opening. When the toner cartridge is moved into the developing unit, the engagement portion is guided in a guide groove formed in the developing unit to move relative to the toner chamber from the closing position to the opening position.