An electrophotographic photosensitive drum unit includes (a) a cylinder having a photosensitive layer at an outer periphery thereof and (b) a drum flange provided at one end of the cylinder, with the drum flange including at least two projections provided inside of the drum flange and projected radially inwardly of the drum flange, and with a space diametrically between the two projections. Each of the two projections projects radially inwardly of the drum flange to a greater extent at a position farther from the cylinder than it does at a position closer to the cylinder.