The invention provides a disk cartridge having an optical disk (2) rotatably stored in a cartridge main body (5). One surface of the cartridge main body on its side for insertion in a recording and/or reproducing device is in the form of a substantially semicircular arc (7) with its center located at the center of the optical disk stored in the cartridge main body, making it possible to easily discriminate the direction of insertion in the recording and/or reproducing device, and realizing smooth mounting.