A developer cartridge that is attachable to and detachable from a photosensitive drum cartridge and includes a frame, a developer housing section, a developer supplying section, and a guide member is provided. The frame includes a first side wall and a second side wall, and the first side wall and the second side wall extend substantially in a length direction. The developer housing section, for housing a developer, extends from the first side wall to the second side wall in a width direction perpendicular to the length direction. The developer supplying section, for supplying the developer to a photosensitive member, extends from the first side wall to the second side wall in the width direction. The guide member is provided over an outer surface of the developer supplying section, and the guide member being detachable from the developer supplying section. The developer supplying section is formed from a first material, the guide member is formed from a second material, and the first material is different from the second material.