An image forming apparatus is provided. The image forming apparatus includes a housing which comprises a housing side opening through which waste toner passes; a plurality of developing units; a plurality of toner cartridges which correspond to the plurality of developing units comprising a first containing unit for containing the toner to be supplied to the toner carrier; wherein at least one of the toner cartridges comprises a second containing unit for containing the waste toner, the second containing unit comprises a cartridge side opening for receiving the waste toner having passed through the housing side opening, and wherein the housing side opening and the cartridge side opening are disposed to be opposite to each other along the substantially horizontal direction.