A magnetic disk cartridge capable of being loaded into a disk drive comprises a housing, and a magnetic disk, which is accommodated for rotation within the housing. The housing is provided with an opening, which allows a magnetic head of the disk drive to access a surface of the magnetic disk, and a shutter for opening and closing the opening. The housing has an outer peripheral shape composed of a circular arc, which is formed along a contour of the magnetic disk, and two straight lines, which are circumscribed with the circular arc formed along the contour of the magnetic disk and are normal to each other.