Abstract: A magnetic disk cartridge has a magnetic head access opening formed in one side surface of a flat cassette shell having a top surface, a bottom surface and side surfaces. A shutter member provided to be slidable on the cassette shell for opening and closing the magnetic head access opening is constituted as a sectionally .hoarfrost.-shaped member formed by interconnecting a pair of upper wings, a shutter section and a pair of lower wings that extend respectively along the top surface, one side surface and the bottom surface. Portions of the top surface and the bottom surface of the cassette shell other than those adjacent to the one side surface portion formed with the magnetic head access opening are formed with recesses for receiving and guiding the upper wings and the lower wings of the shutter member.