A drum unit usable in a process cartridge includes a photosensitive drum having an axis L1 and a coupling member having an axis L2. The coupling member includes a first end portion connected to the photosensitive drum and a second end portion, with the second end portion including at least one projection that is open to the axis L2 and an outer surface that faces away from the first end portion. For least part of the outer surface, a maximum distance from the axis L2 to the outer surface along a line perpendicular to the axis L2 decreases as the distance along the axis L2 from the first end portion increases.