An image forming apparatus includes a housing, developing units disposed in parallel with each other in the housing and including an image carrier, and a toner carrier to supply toner to the image carrier. Toner cartridges that correspond to a respective developing unit when each of the toner cartridges is attached to the housing are configured to be attached to the housing. Each toner cartridge includes a first containing unit configured to contain toner. The apparatus also includes a belt unit disposed to oppose the image carriers and a collecting unit to contact a surface of the belt to collect waste toner. A first toner cartridge includes a second containing unit configured to contain waste toner collected by the collecting unit. Each of the toner cartridges other than the first toner cartridge does not include a second containing unit.