An electrophotographic photosensitive drum unit which is detachably mountable to a main assembly of an electrophotographic image forming apparatus including a driving shaft having a rotational force applying portion by moving in a mounting direction substantially perpendicular to an axis of said driving shaft, 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 engageable with said rotational force applying portion to receive a rotational force for rotating said electrophotographic photosensitive drum, said coupling member being capable of taking a rotational force transmitting angular position for transmitting the rotational force for rotating said electrophotographic photosensitive drum to said electrophotographic photosensitive drum, a pre-engagement angular position in which said coupling member is inclined away from the axis of said electrophotographic photosensitive drum and from said rotational force transmitting angular position downstream in the mounting direction of said drum unit and a disengaging angular position in which said coupling member is inclined away from the axis of said electrophotographic photosensitive drum and from said rotational force transmitting angular position; iii) a regulating portion for regulating an inclination angle of said coupling member such that a downward inclination angle of said coupling member is smaller than an inclination angle of said coupling member when said coupling member is at the pre-engagement angular position; wherein in mounting said drum unit to the main assembly of the apparatus by moving said drum unit in the mounting direction substantially perpendicular to the axis of said electrophotographic photosensitive drum, said coupling member moves from the pre-engagement angular position to the rotational force transmitting angular position to oppose the driving shaft, and in dismounting said drum unit from the main assembly of the apparatus by moving said drum unit in a dismounting direction, which is opposite to the mounting direction, substantially perpendicular to the axis of said electrophotographic photosensitive drum, said coupling member moves from the rotational force transmitting angular position to the disengaging angular position to disengage from the driving shaft, wherein said regulating portion surrounds said coupling member in the mounting direction perpendicular to the axis of said electrophotographic photosensitive drum, and characterised in that said regulating portion is provided with a first arcuate portion (170a, 170g) and a projected portion projecting in the mounting direction continuing from the first arcuate portion (170a, 170g), wherein said first arcuate portion (170a, 170g) regulates the downward inclination of said coupling member, and said projected portion regulates the inclination angle of the coupling member in the pre-engagement angular position. i) an electrophotographic photosensitive drum rotatable about an axis and having a photosensitive layer at its peripheral surface; ii) a coupling member engageable with said rotational force applying portion to receive a rotational force for rotating said electrophotographic photosensitive drum, said coupling member being capable of taking a rotational force transmitting angular position for transmitting the rotational force for rotating said electrophotographic photosensitive drum to said electrophotographic photosensitive drum, a pre-engagement angular position in which said coupling member is inclined away from the axis of said electrophotographic photosensitive drum and from said rotational force transmitting angular position downstream in the mounting direction of said drum unit and a disengaging angular position in which said coupling member is inclined away from the axis of said electrophotographic photosensitive drum and from said rotational force transmitting angular position; iii) a regulating portion for regulating an inclination angle of said coupling member such that a downward inclination angle of said coupling member is smaller than an inclination angle of said coupling member when said coupling member is at the pre-engagement angular position; wherein in mounting said drum unit to the main assembly of the apparatus by moving said drum unit in the mounting direction substantially perpendicular to the axis of said electrophotographic photosensitive drum, said coupling member moves from the pre-engagement angular position to the rotational force transmitting angular position to oppose the driving shaft, and in dismounting said drum unit from the main assembly of the apparatus by moving said drum unit in a dismounting direction, which is opposite to the mounting direction, substantially perpendicular to the axis of said electrophotographic photosensitive drum, said coupling member moves from the rotational force transmitting angular position to the disengaging angular position to disengage from the driving shaft, wherein said regulating portion surrounds said coupling member in the mounting direction perpendicular to the axis of said electrophotographic photosensitive drum, and characterised in that said regulating portion is provided with a first arcuate portion (170a, 170g) and a projected portion projecting in the mounting direction continuing from the first arcuate portion (170a, 170g), wherein said first arcuate portion (170a, 170g) regulates the downward inclination of said coupling member, and said projected portion regulates the inclination angle of the coupling member in the pre-engagement angular position.