An optical disk (10) is disclosed which records information that can be optically read. The disk has a spiral guide track (14). The disk further includes a single index field (20) and a plurality of sectors (18). The index field includes an index mark (22) that indicates the position at which an optical head is caused to jump for repeated trace of a specific portion of the guide track. Each of the sectors has a sector head (26) and a data field (30) that comes following the sector head. The sector head comprises a sector mark (26) located at the head of the sector and an address field (28). This address field contains a track address, a sector address, and an error detecting and correcting code. The index mark in the index field is detected, and the optical head is made to perform a jumping operation in response to the resultirg signal, whereby the jumping operation is rendered certain. When the address fields are being read, if some sector addresses cannot be read due to drop-out or for other cause, tie signal produced by detecting the index marks are counted by a counter into which the signal produced by detecting the sector marks is clocked, for preparing the sector addresses.