A rigid disk includes a transparent glass substrate having guide grooves for a laser beam for an optical disk which grooves are formed in one surface of the substrate, a reflective film formed on the lower surface of the substrate, a light-transmitting magnetic oxide film formed on the other surface of the substrate which is flat and smooth, and a lubricating layer formed on the magnetic oxide film. The magnetic oxide film has only a spinel-type crystal structure, and contains iron and cobalt. A magnetic head and an optical head are mounted on an arm movable back and forth relative to the rigid disk in a radial direction of the rigid disk. A laser beam emitted from the optical head passes through the lubricating layer, the magnetic oxide film and the substrate of the rigid disk, and is reflected by the reflective film to be returned to the optical head, thereby detecting a track signal formed by a configuration of the grooves by the optical head, so that the driving of the arm is controlled so as to move the magnetic head along a track on a recording surface of the rigid disk in a servo-controlled manner.