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 bearingmember and a pair of side plates. The developer container is configured to contain a developer including toner and carriers, and includes the toner inlet mounted at a predetermined position on a top surface of the developer container and configured to receive toner into the developer container. The pair of side plates are arranged in parallel with a predetermined distance which determines a predetermined longitudinal width of the process cartridge and are configured to support the components in the process cartridge. The toner storage stores toner. The toner supplying unit is configured to convey the toner from the toner storage to the developer container through the toner inlet.