An electrophotographic image forming apparatus comprising: a body comprising an opening and a mounting rail; a photoreceptor cartridge attachable to, or detachable from, the body through the opening and being adapted to be guided by the mounting rail, and comprising a photoconductive drum and a mounting portion; a development cartridge attachable to, or detachable from, the mounting portion through the opening while the photoreceptor cartridge is mounted in the body, and comprising a development roller for supplying toner to an electrostatic latent image formed on the photoconductive drum, and a cover for opening or closing the opening; wherein the mounting portion comprises first and second guide rails (231,232), the development cartridge further comprises first and second guide protrusions (310,320) disposed on both side portions thereof and supported by the first and second guide rails (231,232), respectively when the development cartridge is attached to or detached from the mounting portion, the first and second guide rails (231,232) do not lock the first and second guide protrusions (310,320) when the development cartridge is attached to the mounting portion, the cover comprises a pressurizing unit for fixing the development cartridge to the mounting portion by providing a pressing force to the development cartridge in a direction parallel to a center line connecting centers of the photoconductive drum and development roller while the opening is closed, and the development cartridge is fixed to the mounting portion by the pressing force when the cover is closed, and the development cartridge is detachable from the mounting portion without unlocking the development cartridge from the mounting portion when the cover is opened; characterized in that the pressurizing unit comprises a first pressurizing unit and a second pressurizing unit respectively providing a first pressing force and a second pressing force to the development cartridge by being spaced apart from the center line in opposite directions respectively by a first distance and a second distance, wherein the first pressing force and the second pressing force have directions parallel to the center line, the first pressing force generates a moment in an opposite direction to a moment of rotation generated by rotation of the development roller, and the second pressing force generates a moment in the same direction as the moment of rotation generated by rotation of the development roller, wherein, in order to compensate for the moment of rotation and a moment of rotation by the second pressing force, the apparatus satisfies at least one of the conditions that: i) the first pressing force is larger than the second pressing force, and ii) the first distance is longer than the second distance. a body comprising an opening and a mounting rail; a photoreceptor cartridge attachable to, or detachable from, the body through the opening and being adapted to be guided by the mounting rail, and comprising a photoconductive drum and a mounting portion; a development cartridge attachable to, or detachable from, the mounting portion through the opening while the photoreceptor cartridge is mounted in the body, and comprising a development roller for supplying toner to an electrostatic latent image formed on the photoconductive drum, and a cover for opening or closing the opening; wherein the mounting portion comprises first and second guide rails (231,232), the development cartridge further comprises first and second guide protrusions (310,320) disposed on both side portions thereof and supported by the first and second guide rails (231,232), respectively when the development cartridge is attached to or detached from the mounting portion, the first and second guide rails (231,232) do not lock the first and second guide protrusions (310,320) when the development cartridge is attached to the mounting portion, the cover comprises a pressurizing unit for fixing the development cartridge to the mounting portion by providing a pressing force to the development cartridge in a direction parallel to a center line connecting centers of the photoconductive drum and development roller while the opening is closed, and the development cartridge is fixed to the mounting portion by the pressing force when the cover is closed, and the development cartridge is detachable from the mounting portion without unlocking the development cartridge from the mounting portion when the cover is opened; characterized in that the pressurizing unit comprises a first pressurizing unit and a second pressurizing unit respectively providing a first pressing force and a second pressing force to the development cartridge by being spaced apart from the center line in opposite directions respectively by a first distance and a second distance, wherein the first pressing force and the second pressing force have directions parallel to the center line, the first pressing force generates a moment in an opposite direction to a moment of rotation generated by rotation of the development roller, and the second pressing force generates a moment in the same direction as the moment of rotation generated by rotation of the development roller, wherein, in order to compensate for the moment of rotation and a moment of rotation by the second pressing force, the apparatus satisfies at least one of the conditions that: i) the first pressing force is larger than the second pressing force, and ii) the first distance is longer than the second distance. i) the first pressing force is larger than the second pressing force, and ii) the first distance is longer than the second distance.