An image forming apparatus comprises: a developing device having an outlet from which a two-component developer is partially discharged while the two-component developer is conveyed; a hopper to which (i) a first cartridge containing a first developer including carriers and (ii) a second cartridge containing a second developer including toner and not carriers are attachable, and operable, when the first cartridge is attached thereto, to supply the first developer to the developing device, and when the second cartridge is attached thereto, to supply the second developer to the developing device; and a switching part operable, when the first developer is supplied, to switch the developing device to a first state in which the two-component developer is partially discharged from the outlet, and when the second developer is supplied, to switch the developing device to a second state in which the two-component developer is not discharged from the outlet.