A casette adapter in which an S-casette (first casette) of the professional digital video format is housed and having a shape identical with an different-sized M-casette (second casette) operates without fail under a simple constitution, without accompanying any fear of damage on a tape. The casette adapter capable of housing an S-casette (2) comprises an adapter (1) whose shape is identical to an different-sized M-casette. Reels (5, 6) and a reel brake releasing member (7) of the S-casette (2) are exposed while the S-casette is mounted in the casette adapter. The adaptor (1) includes a rear door (12) capable of opening and closing. The rear door (12) comprises contacts (14) for reading identification signal by contacting the identification terminal of S-casette, wirings (16) for coupling the contacts (14) with an identification terminal (15) provided in a same location as that of M-casette and a lever (18) for opening/closing an anti-erasing window in linkage with the anti-erasing gear of S-casette (2).