An image forming apparatus including cyan (C), magenta (M), yellow (Y), and black (K) developing units using a C developer, a M developer, a Y developer, and a K developer, respectively; at least one auxiliary developing unit using a developer other than the C, M, Y, and K developers; a plurality of photosensitive media formed in the C, M, Y, and K developing units and the auxiliary developing unit, respectively; a plurality of cleaning members formed in the C, M, Y, and K developing units and the auxiliary developing unit, respectively, so as to clean surfaces of the plurality of photosensitive media; and waste developer storage units formed in the C, M, Y, and K developing units and the auxiliary developing unit, respectively, so as to collect waste developer cleaned by the plurality of cleaning members. The developer storage unit and waste developer storage unit of at least one of the C, M, Y, and K developing units, in which there is a large amount of developer consumed and a large amount of waste developer generated, are larger than the developer storage unit and waste developer storage unit of the auxiliary developing unit.