Patent ID: 8041273

Claim:
A photosensitive drum unit comprising: a frame; and a plurality of photosensitive drums supported by the frame and having a shape identical with one another, each of the plurality of photosensitive drums being rotatable about a rotational axis, the plurality of photosensitive drums being arranged such that a distance between rotational axes of adjacent photosensitive drums is a constant distance P, the plurality of drums including a first drum and a second drum, wherein each of the plurality of photosensitive drums has a circumferential reference position that is common to all of the plurality of photosensitive drums, each of the plurality of photosensitive drums having a plurality of marks at circumferential positions that are defined relative to the circumferential reference position, adjacent two positions of the circumferential positions being spaced by a central angle of {(S−P)/S}×360° where S is a circumferential length of each of the plurality of photosensitive drums, each of the first drum and the second drum including both a first mark and a second mark as the plurality of marks, wherein the first drum is positioned such that the first mark of the first drum is set relative to a given angular position, and wherein the second drum is positioned such that the second mark of the second drum is set relative to the given angular position.