A developing device includes a toner cartridge mounting portion, and a toner cartridge to be detachably mounted on the toner cartridge mounting portion. The toner cartridge mounting portion has a bottom wall having a toner acceptance opening formed therein, and a pair of supporting side walls disposed on the opposite side edges of the bottom wall, and an engaging means is disposed on the inner surface of one of the supporting side walls. The toner cartridge includes a generally cylindrical container having an unsealably sealed toner discharge opening formed in the peripheral wall of the container, and a toner accommodated in the container. An engaged means to be engaged with the engaging means is disposed in the outer surface of one side wall of the container. The one side wall of the container is opposed to the inner surface of the one of the supporting side walls to engage the engaging means and the engaged means with each other, and the other side wall of the container is opposed to the inner surface of the other of the pair of supporting side walls, whereby the toner cartridge is mounted on the toner cartridge mounting portion.