A laser printer includes: a main casing with two side walls; lower guiding grooves formed in the side walls; a process cartridge that is guided by the guiding grooves to be mounted in or removed from the main casing; and a lever rotatably supported by a shaft mounted on the main casing at a position near the lower guiding groove in one side wall. When the lever is rotated, a protruding part provided on one end of the lever protrudes into the lower guiding groove and engages with a fixing boss provided on the process cartridge, thereby preventing the process cartridge from moving in a direction opposite the direction in which the cartridge is mounted in the main casing.