A drum unit for a cartridge includes a photosensitive drum and a coupling member having a rotational axis, with the coupling member being operatively connected to the photosensitive drum. The coupling member includes a cylindrical portion having an outer surface and an inner surface that extends about the rotational axis of the coupling member, an arm portion extending from the inner surface of the cylindrical portion, and a projection projecting from the arm portion in a direction toward the rotational axis of the coupling member. The arm portion and the projection are movable relative to the inner surface.