A belt unit is provided with pressure-receiving parts, and a photosensitive unit is provided with pressing parts that applies pressing force to the pressure-receiving parts when the photosensitive unit is inserted to a main body of an image forming device along a guide groove formed in the main body. The pressing force applied to the pressure-receiving parts presses the belt unit toward a regular mounting position in the main body, so that the mounting of the belt unit is completed at the same time the mounting of the photosensitive unit is completed.