A cartridge that can be accurately positioned and stably held with respect to a main body of an electrophotographic image forming apparatus without causing a deterioration in operability for mounting and detaching the cartridge from the apparatus main body even if a reduction in cost and a reduction in size of the apparatus main body are realized and an electrophotographic image forming apparatus including such cartridge. The cartridge is a drum cartridge that has an electrophotographic photosensitive drum and is detachably mountable to the apparatus main body. The drum cartridge has lock portions that lock on both side plates of the apparatus main body at both ends in a longitudinal direction of the drum cartridge such that the apparatus main body follows the drum cartridge mounted on the apparatus main body.