An image forming apparatus includes a main frame, a plurality of process cartridges, a cartridge-supporting body, a belt, a waste toner cartridge, a contact-separation mechanism, and a collective conveying unit. Each process cartridge includes a photosensitive drum and a drum cleaning unit collecting waste toner on the drum. The cartridge-supporting body is movable between an internal position inside the frame and an external position outside the frame. The belt confronts the process cartridges in the internal position. The waste toner cartridge accommodates the waste toner collected from the drums. The contact-separation mechanism moves the cartridge-supporting body between a contact position where the drums are in contact with the belt and a separated position where the drums are out of contact with the belt. The collective conveying unit aggregates waste toner collected from drums and conveys collectively the aggregated waste toner to the waste toner cartridge.