A method for recording a unique ID to a read-only optical disk adopts the 1-7 PP modulation technique and provides in a plurality of predetermined positions postscript areas each having a predetermined postscript pattern. The postscript pattern takes a form of 3T (pit)-2T (land)-3T (pit) and has a length of 8T (largest code length of a modulated bit string). Modulated bit strings before and after the 3T-2T-3T postscript pattern are formed so that the pit-land-pit postscript pattern complies with the rule of the variable-length modulation even when it is replaced as a whole with a pit-only pattern. Further, the land in the middle will not physically change when irradiated with laser light having a reading-level power, but will be melted with a power higher than the reading-level one.