The present invention is for the purpose of allowing a transferring mechanism to certainly carry out the direct insertion/extraction of a cartridge into/from a recording/reproduction unit. For this purpose, in a structure according to this invention, introduction guides (a guide member) for guiding the cartridge to be inserted from the external through an accessor (the transferring mechanism) toward the interior of a deck (the recording/reproduction unit) is provided in a cartridge insertion/discharge opening section of the deck. The introduction guides are formed to have tapered surfaces broadening out from the deck to the external. Further, this invention is applicable to a large-capacity external storage memory storing a large number of cartridges such as magnetic tape cartridges and optical disk cartridges.