ck 100 is structured so as to include a short mark recording area 101 recording a short mark (having a mark length of 2T where T is a channel bit length), a long mark recording area 102 recording a long mark (having a mark length of 8T where T is a channel bit length), and a data recording area 103 recording digital data. [0030] Referring to FIGS. 2, 3(a), and 3(b), the following will explain the