A drum unit for a cartridge includes a cylindrical photosensitive drum having an axis L1 and a coupling member operatively connected to the photosensitive drum. The coupling member includes a first member having a cylindrical portion, a part of the first member being positioned inside of the photosensitive drum and a part of the first member being positioned outside of the photosensitive drum. The coupling member also includes a second member operatively connected to first member, the second member including a plurality of arm portions that are movable relative to the first member, with at least a part of each of the arm portions being positioned inside of the photosensitive drum and inside of the first member.