According to the present invention, there is provided a disk cartridge capable of being made compact although the disc cartridge has double structure, which cartridge is improved in light blocking effect and dustproof property, and is particularly suitable for use as a media disk. In this disk cartridge, a shutter is slidably mounted between an inner case and an outer case, and a joint surfaces of upper and lower halves of the inner case, and joint surfaces of upper and lower halves of the outer case are provided at positions at different heights. Light incidence preventing parts which consist of sets of a plurality of lines of ribbed projections are formed on an inner surface of the upper half and an inner surface of the lower half of the outer case.