An image forming apparatus is provided including a housing having a first opening and a second opening which are directed in a same direction; a plurality of developing units which are disposed in parallel with each other in the housing along an oblique direction inclined with respect to a horizontal plane, the developing units which are configured to be attached to and detached from the housing along the oblique direction through the first opening; and a plurality of developer cartridges which correspond to the plurality of developing units, each of the developer cartridges being disposed in parallel with each other in the housing to be opposite to an end of a respective one of the developing units along a substantially horizontal direction in a longitudinal direction of the developing units, the developer cartridges which are configured to be attached to and detached from the housing through the second opening.