Provided is a rear converter optical system which is detachably mounted on an image side of a main lens system to increase a focal length of the main lens system, in which an effective diameter Φim of an imaging area of the main lens system, an effective diameter Φcr of a lens surface closest to the image side of the rear converter optical system, a lateral magnification βc of the rear converter optical system in the case where the rear converter optical system is mounted on the main lens system, a backfocus skc in the case where the rear converter optical system is mounted on the main lens system are appropriately set.