An image forming apparatus includes a process cartridge, a toner storage, and a toner supplying unit. The process cartridge includes an image bearing member, a developer container, a developer bearing member, and a pair of side plates. The developer container includes a toner inlet mounted at a predetermined position on one of its top surface. The pair of side plates are arranged in parallel and separated by a predetermined distance which determines a predetermined longitudinal width of the process cartridge and configured to support the components in the process cartridge.