In an image forming apparatus, each of a plurality of image forming units includes a receiving member formed with a reception opening configured to receive developer. Each of a plurality of developer cartridges includes a supply member formed with a supply opening through which developer is configured to be supplied to a corresponding one of the plurality of image forming units. A cartridge supporting member is mounted in a main casing and configured to move between a first position and a second position. The supply opening and the reception opening are in communication with each other when the cartridge supporting member is in the first position and the cartridge supporting member is pulled outside the main casing when the cartridge supporting member is in the second position. One of the receiving member and the supply member protrudes toward remaining one, and the remaining one is configured to receive the one.