The present invention provides a cartridge accommodating a disk therein which is inserted into a cartridge holder, the cartridge holder being provided with a mis-insertion protection member. The mis-insertion protection member is located within an insertion path along which the cartridge is inserted into the cartridge holder. The cartridge has a substantially rectangular shape. The cartridge has a corner portion at each of four corners of the substantially rectangular shape, the corner portion being shaped so as to abut on the mis-insertion protection member to move the mis-insertion protection member out of the insertion path. The cartridge has at least two mis-insertion protection mechanisms for engaging with the mis-insertion protection member so as to prevent the cartridge from being mis-inserted. The cartridge has two side surfaces which are substantially perpendicular to a surface of the disk and extend along a direction substantially parallel to a direction in which the cartridge is inserted into the cartridge holder. A mis-insertion protection mechanism of the at least two mis-insertion protection mechanisms is provided on each of the side surfaces.
