An image forming apparatus includes a housing; a partition wall which is provided in the housing along a substantially vertical direction, the partition wall partitioning an inner space of the housing into a first space and a second space; developing units which are disposed in parallel along the substantially vertical direction in the first space, each of the developing units comprising a first reception opening and a return opening; and developer cartridges which correspond to the plurality of developing units, the developer cartridges being disposed in parallel along the substantially vertical direction in the second space, each of developer cartridges comprising a supply opening and a second reception opening. The first reception openings of the developing units communicate with the supply openings of respective ones of the developer cartridges, and the return openings of the developer units communicate with the second reception openings of respective ones of the developer cartridges.