A cartridge is detachably mountable to the main assembly of an electrophotographic image forming apparatus. The cartridge comprises a developer accommodating portion for accommodating a developer, and a developing roller for developing an electrostatic latent image formed on an electrophotographic photosensitive drum with the developer. A rotational force receiving member is provided for receiving a rotational force for rotating the developing roller. A side cover is provided with an elastic member and an inclination regulating portion for regulating an inclination of the rotational force receiving member. The side cover is mounted to a cartridge frame by the fastening member, with the rotational force receiving member being mounted to the cartridge frame integrally with the side cover.