Patent ID: 6906990

Claim:
A phase-change optical disk having a plurality of sectors forming tracks which are arranged spirally or concentrically, the plurality of sectors each comprising: a recording region for recording user data by randomly shifting the recording start position of the user data every time the user data is recorded; and a header region having address information concerning the position of each sector; the recording region comprising: a first region used for a first purpose; a second region used for a second purpose differing from the first purpose, the second region being continuous from the first region; a data region for recording the user data; and a synchronization mark provided between the second region and the data region, the synchronization mark being detected to acquire synchronization with the data region when reading the user data from the data region, wherein a boundary between the first region and the second region is detected prior to detecting the synchronization mark, the synchronization mark is detected in accordance with the position of the detected boundary between the first region and the second region, and the user data is obtained from the data region in accordance with the detected synchronization mark.