An electrophotographic image forming apparatus for forming an image on a recording material includes an electrophotographic photosensitive drum, a charger for charging the photosensitive drum, a developer for developing a latent image formed on the photosensitive drum into a toner image, a transferrer for transferring the toner image onto the recording material, a fixer for fixing the toner image on the recording material, a motor, a main assembly side gear for receiving a driving force from the motor, and a hole formed at the longitudinal end of the photosensitive drum. The hole has a polygonal cross-section. The apparatus also includes a projection formed substantially at a center of the gear. When the main assembly side gear rotates with the hole and projection engaged with each other, a rotational driving force is transmitted from the gear to the photosensitive drum through the hole and the projection with the projection is pulled into the hole. An end portion of the gear having the projection and a end surface defining the hole contact each other so that the process cartridge is positioned in place in a longitudinal direction of the photosensitive drum relative to the main assembly. The apparatus also includes a mover for imparting relative movement between the hole and the photosensitive drum to engage the projection with the hole.