Abstract: A process cartridge detachably mountable to a main assembly of an electrophotographic image forming apparatus includes an electrophotographic photosensitive member; process members actable on the electrophotographic photosensitive member; an exposure zone, provided in a cartridge frame, for exposing a part of the electrophotographic photosensitive member to permit, when the process cartridge is mounted to the main assembly of the apparatus, a detector provided in the main assembly to detect a density of a toner image formed on the electrophotographic photosensitive member, to project light emitted from the detector to the electrophotographic photosensitive member on which the toner image is formed, and to direct the light reflected by the electrophotographic photosensitive member to the detector.

Abstract: A developing device which is capable of being detachably mounted to a rotary unit of an image forming apparatus and which includes a developer carrying member for forming a development region together with an image carrying member and rotating as carrying a developer, a developer containing section for containing the developer to be supplied to the developer carrying member, and a pressure relief portion provided in a surface substantially perpendicular to the rotational axis of the rotary unit, of the developer containing section.