Abstract: A disk cartridge comprises a case including an upper and lower shell halves for accommodating a disk and an opening portion provided on each shell half, so as to be opened and closed by a shutter. Each shell half is formed by an injection molding of polycarbonate in which a mold having a single injection hole is used. The injection hole is formed in the mold, at a position opposed to a center of a groove in its longitudinal direction on a surface of the case for receiving a retaining plate to guide a front portion of a slide plate of the shutter. The injection hole has a diameter of about 1.5 mm in case of a 5-inch disk cartridge.