A disk cartridge of the present invention comprises a case body (101) having a disk-storage portion and an opening/closing cover (201) having a pair of disk holding members (202) that hold a disk (10) in its inplane direction. The accommodated disk (10) is loaded together with the opening/closing cover (201) into an adapter having compatibility with a large disk cartridge. Thus, information can be, for example, reproduced from the disk (10) in a drive unit designed for a large disk cartridge via the adapter. Moreover, a series of operations can be conducted without touching the disk directly.