A developer cartridge that is attachable to and detachable from a photosensitive body cartridge that includes a photosensitive member is provided. The developer cartridge includes a frame that includes a first side wall and a second side wall. The developer cartridge also includes a developer supplying section for supplying a developer to the photosensitive body cartridge and a developer housing section for housing the developer. The developer housing section includes an open end at which the developer housing section adjoins the developer supplying section, and a closed end substantially opposite from the open end. The closed end includes an exterior surface that extends from a substantially bottommost portion of the developer housing section to a top portion of the developer housing section. At least one engaging portion of the developer cartridge extends outward from the exterior surface of the developer housing section. When the developer cartridge is detachably attached to the photosensitive body cartridge, the engaging portion engages with the photosensitive body cartridge to press the developer supplying section towards the photosensitive body cartridge and to regulate movement of the developer cartridge.