A cartridge adaptor accommodates a second cartridge which is smaller than a first cartridge so as to allow the second cartridge to be mounted in a disk apparatus designed to receive the first cartridge. The cartridge adaptor includes a shutter opener engageable with a shutter opener of the disk apparatus to open and close a shutter of the second cartridge, a projection engageable with a recess of the second cartridge to restrict insertion direction of the second cartridge into the disk apparatus, and a conveying member which is too be inserted into a transmission aperture formed in correspondence with an identification hole formed in the second cartridge, thereby opening and closing a detection hole of the cartridge adaptor formed at a corresponding position to an identification hole of the first cartridge. When the cartridge adaptor accommodating the second cartridge is mounted in the disk apparatus, the shutter of the second cartridge is opened, and the information on a disk housed in the second cartridge provided by the identification hole is correctly detected by the disk apparatus.