An electrophotographic photosensitive drum unit usable with a main assembly of an electrophotographic image forming apparatus, the main assembly including a driving shaft, to be driven by a motor, said driving shaft having a rotational force applying portion, wherein said electrophotographic photosensitive drum unit is mountable to the main assembly in a mounting direction substantially perpendicular to an axial direction of the driving shaft, wherein said drum unit is dismountable from the main assembly in a dismounting direction opposite to the mounting direction, said electrophotographic drum unit comprising: i) an electrophotographic photosensitive drum having a photosensitive layer at a peripheral surface thereof, said electrophotographic photosensitive drum being rotatable about an axis thereof; 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, and a pre-engagement angular position in which said coupling member is inclined away from the axis of said electrophotographic photosensitive drum from said rotational force transmitting angular position, and a disengaging angular position in which said coupling member is inclined away from the axis of said electrophotographic photosensitive drum from said rotational force transmitting angular position, wherein said electrophotographic photosensitive drum unit is adapted such that when said electrophotographic drum unit is mounted to the main assembly of the electrophotographic image forming apparatus in the mounting direction substantially perpendicular to the axis of said electrophotographic photosensitive drum, said coupling member moves from said pre-engagement angular position to said rotational force transmitting angular position, and wherein said electrophotographic photosensitive drum unit is adapted such that when said electrophotographic drum unit is dismounted from the main assembly of the electrophotographic image forming apparatus in the dismounting direction substantially perpendicular to the axis of said electrophotographic photosensitive drum, said coupling member moves from said rotational force transmitting angular position to said disengaging angular position. i) an electrophotographic photosensitive drum having a photosensitive layer at a peripheral surface thereof, said electrophotographic photosensitive drum being rotatable about an axis thereof; 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, and a pre-engagement angular position in which said coupling member is inclined away from the axis of said electrophotographic photosensitive drum from said rotational force transmitting angular position, and a disengaging angular position in which said coupling member is inclined away from the axis of said electrophotographic photosensitive drum from said rotational force transmitting angular position,