A photosensitive member cartridge, having a photosensitive drum, is provided with guide grooves formed in opposite side walls of the photosensitive member cartridge. The guide grooves extend in an accurate shape from an upwardly open accommodating portion to a vicinity of a shaft of the photosensitive drum. A shaft of a developing roller has protruded opposite end portions that protrude outward from opposite sides of a case of a developer cartridge. The protruded end portions of the developing roller shaft are provided with bearings. When the developer cartridge is inserted into the accommodating portion toward the photosensitive drum, the guide grooves support and guide the bearings on the end portions of the developing roller shaft, so that the developer cartridge becomes properly set in the accommodating portion. An inverted triangular-shaped action-receiving portion, protruding from the outer surface of one of the opposite sides of the developer cartridge, is restrained from above by a lower contact portion of a lock lever that is disposed in the accommodating portion.