through the effect of heat, for example. By making the length of one information recording unit provided when information is recorded on a track on the inner periphery of the optical disk equal to the length thereof provided when information is recorded on a track on the outer periphery of the optical disk (keeping the constant recording line density in both cases) in the track direction, it is possible to obtain more recording capacity of information to be recorded on one optical disk as compared with the recording format used in the magnetic disk located in the harddisk drive. The recording format used in the magnetic disk of the harddisk drive has as one information recording unit an arc length provided when an angle to an arc becomesconstant. The length of one information recording unit is made variable according to the radial location of the disk. That is, the recording density is made lower toward the outer periphery.
Recently, a rewritable and recordable optical disk is generally a phase-change optical disk if the disk has a large capacity. This optical disk is constructed by forming amorphous marks on a crystalline area served as a recording layer of the optical disk through the use of heat of the condensed, or focused laser beam and then obtaining information as the corresponding signal through a difference of reflectivity between the amorphous portion and the crystalline portion. Since the marks are formed by heat, it is necessary to radiate the laser in consideration of heat conduction and diffraction on the