A cartridge detachably mountable to a main assembly of an electrophotographic image forming apparatus, includes a developer accommodating portion for accommodating a developer; a developing roller for developing an electrostatic latent image formed on an electrophotographic photosensitive drum with the developer accommodated in the developer accommodating portion; a cartridge frame extended in a longitudinal direction of the developing roller; a bearing member, provided at one longitudinal end portion of the cartridge frame, for supporting a developing roller shaft portion at one longitudinal end portion of the developing roller; a rotational force receiving member for receiving, from the main assembly, a rotational force for rotating the developing roller, in a state in which the cartridge is mounted to the main assembly; a side cover provided outside the bearing member with respect to a longitudinal direction of the cartridge frame; a first fastening member fastening the bearing member to the cartridge frame to mount the bearing member to the cartridge frame; and a second fastening member fastening the side cover to the cartridge frame to mount the side cover to the cartridge frame.