Provided are: a first entry preventing nail that interferes with a cartridge at a position closer to an insertion/extraction port than an insertion completed position of an insertion-side end face of the cartridge inserted first; a second entry preventing nail at a position with a space from the first entry preventing nail slightly longer than length of the cartridge, which allows entry of the cartridge and retreat of the first entry preventing nail at that position when a tapered face of the cartridge is at that position while preventing the entry of the cartridge and retreat of the first entry preventing nail at that position when the tapered face of the cartridge is not at that position; and a linkage module which allows advancing and retreating actions of the first and second entry preventing nails in a tradeoff relation.