A rotating force transmitting part is provided for an electrophotographic photosensitive drum for a main assembly of the electrophotographic image forming apparatus. The main assembly includes a motor-driven shaft having a rotating force applier. The drum is dismountable from the main assembly in a direction substantial perpendicular with an axial direction of the shaft. The part includes a coupler engageable with the force applier to receive a rotational force for rotating the drum when mounted to the main assembly. The coupler moves between a position for transmitting the rotational force for rotating the drum to the drum and a disengaged position inclined away from the axis of the drum. When the cartridge is dismounted from the main assembly substantially perpendicular to the axis of the drum, the coupler moves from the transmitting position to the disengaging position.