An electrophotographic image forming apparatus comprising: a body having an opening; a photoreceptor cartridge operable to be attached to, or detached from, the body through the opening, and comprising a mounting portion; a development cartridge; and a cover operable to open, or close, the opening, and comprising a pressurization portion for fixing the development cartridge to the mounting portion, wherein, when the cover is closed, the development cartridge is fixed to the photoreceptor cartridge by being pressurized by the pressurization portion, wherein, when the photoreceptor cartridge is mounted in the body and the development cartridge is fixed to the photoreceptor cartridge, the fixing of the development cartridge to the photoreceptor cartridge is released only by opening the cover, and wherein the development cartridge is operable to be attached to, or detached from, the mounting portion through the opening in a state where the photoreceptor cartridge is mounted in the body; wherein the mounting portion comprises first and second guide members respectively extending from both side portions of the photoreceptor cartridge towards the opening to support both side portions of the development cartridge, wherein the mounting portion comprises guide rails provided at each of the first and second guide members (210, 220), and first and second accommodation portions (241, 242), characterised in that first and second guide protrusions (310, 320) guided by the guide rails and respectively accommodated in the first and second accommodation portions (241, 242) are provided respectively at the both side portions of the development cartridge, wherein that the guide rails comprise a first guide rail for guiding the first guide protrusion to the first accommodation portion, and a second guide rail for guiding the second guide protrusion to the second accommodation portion by being branched from the first guide rail, wherein a protrusion amount of the second guide protrusion from both side walls of the development cartridge is smaller than a protrusion amount of the first guide protrusion and wherein the first and second guide rails (231, 232) each have a rib shape respectively protruding from inner walls of the first and second guide members (210, 220), and a protrusion amount of the first guide rail from the inner wall is smaller than a protrusion amount of the second guide rail near a branching location of the second guide rail. a body having an opening; a photoreceptor cartridge operable to be attached to, or detached from, the body through the opening, and comprising a mounting portion; a development cartridge; and a cover operable to open, or close, the opening, and comprising a pressurization portion for fixing the development cartridge to the mounting portion, wherein, when the cover is closed, the development cartridge is fixed to the photoreceptor cartridge by being pressurized by the pressurization portion, wherein, when the photoreceptor cartridge is mounted in the body and the development cartridge is fixed to the photoreceptor cartridge, the fixing of the development cartridge to the photoreceptor cartridge is released only by opening the cover, and wherein the development cartridge is operable to be attached to, or detached from, the mounting portion through the opening in a state where the photoreceptor cartridge is mounted in the body; wherein the mounting portion comprises first and second guide members respectively extending from both side portions of the photoreceptor cartridge towards the opening to support both side portions of the development cartridge, wherein the mounting portion comprises guide rails provided at each of the first and second guide members (210, 220), and first and second accommodation portions (241, 242), characterised in that first and second guide protrusions (310, 320) guided by the guide rails and respectively accommodated in the first and second accommodation portions (241, 242) are provided respectively at the both side portions of the development cartridge, wherein that the guide rails comprise a first guide rail for guiding the first guide protrusion to the first accommodation portion, and a second guide rail for guiding the second guide protrusion to the second accommodation portion by being branched from the first guide rail, wherein a protrusion amount of the second guide protrusion from both side walls of the development cartridge is smaller than a protrusion amount of the first guide protrusion and wherein the first and second guide rails (231, 232) each have a rib shape respectively protruding from inner walls of the first and second guide members (210, 220), and a protrusion amount of the first guide rail from the inner wall is smaller than a protrusion amount of the second guide rail near a branching location of the second guide rail.