An electrophotographic photosensitive drum unit usable with a process cartridge, said drum unit comprising: i) an electrophotographic photosensitive drum rotatable about an axis and having a photosensitive layer at its peripheral surface; ii) a coupling member for receiving an external force for rotating said electrophotographic photosensitive drum, wherein said coupling member is capable of taking a first angular position for transmitting the external force to said electrophotographic photosensitive drum, a second angular position in which an axis of said coupling member is inclined away from the axis of said electrophotographic photosensitive drum and from the first angular position, and a third angular position (α8) inclined away from the axis of said electrophotographic photosensitive drum and from the first angular position; and iii) a regulating portion for regulating an inclination angle of said coupling member such that a downward inclination angle of said coupling member at said third angular position (α8) is smaller than an inclination angle of said coupling member when said coupling member is at the second angular position, wherein said regulating portion surrounds said coupling member in a perpendicular direction perpendicular to the axis of said electrophotographic photosensitive drum, and said regulating portion is provided with a first arcuate portion (170a, 170g) and a projected portion projecting in the perpendicular direction continuing from the first arcuate portion (170a, 170g), and wherein said first arcuate portion (170a, 170g) regulates the downward inclination of said coupling member at the third angular position (α8), and said projected portion regulates the inclination angle of the coupling member in the second angular position, wherein said first arcuate portion (170a, 170g) is provided with a regulating projection projecting in the axial direction from said first arcuate portion (170a, 170g), and the regulating projection is provided with a second arcuate portion having the same radius of arc as that of said first arcuate portion (170a, 170g), characterised by a flat surface portion extending from said second arcuate portion toward said projected portion, and by the regulating portion being configured such that when said coupling member receives a second external force different from said external force from the main assembly of the apparatus, said coupling member is moved by the external force along said second arcuate portion and said flat surface portion to said projected portion, and said coupling member is positioned at the second angular position. i) an electrophotographic photosensitive drum rotatable about an axis and having a photosensitive layer at its peripheral surface; ii) a coupling member for receiving an external force for rotating said electrophotographic photosensitive drum, wherein said coupling member is capable of taking a first angular position for transmitting the external force to said electrophotographic photosensitive drum, a second angular position in which an axis of said coupling member is inclined away from the axis of said electrophotographic photosensitive drum and from the first angular position, and a third angular position (α8) inclined away from the axis of said electrophotographic photosensitive drum and from the first angular position; and iii) a regulating portion for regulating an inclination angle of said coupling member such that a downward inclination angle of said coupling member at said third angular position (α8) is smaller than an inclination angle of said coupling member when said coupling member is at the second angular position, wherein said regulating portion surrounds said coupling member in a perpendicular direction perpendicular to the axis of said electrophotographic photosensitive drum, and said regulating portion is provided with a first arcuate portion (170a, 170g) and a projected portion projecting in the perpendicular direction continuing from the first arcuate portion (170a, 170g), and wherein said first arcuate portion (170a, 170g) regulates the downward inclination of said coupling member at the third angular position (α8), and said projected portion regulates the inclination angle of the coupling member in the second angular position, wherein said first arcuate portion (170a, 170g) is provided with a regulating projection projecting in the axial direction from said first arcuate portion (170a, 170g), and the regulating projection is provided with a second arcuate portion having the same radius of arc as that of said first arcuate portion (170a, 170g), characterised by a flat surface portion extending from said second arcuate portion toward said projected portion, and by the regulating portion being configured such that when said coupling member receives a second external force different from said external force from the main assembly of the apparatus, said coupling member is moved by the external force along said second arcuate portion and said flat surface portion to said projected portion, and said coupling member is positioned at the second angular position.