PURPOSE: To make it possible to attain random access to a desired data block easily by providing an unrecorded area, equivalent to at least one track or more, to each block on an optical disk.

CONSTITUTION: On optical disk 1, the 1st data block 2, for example, is recorded to a length equivalent to two rotations of disk 1 and the 2nd data block 4, on the other hand, is recorded to a length equivalent to one rotation. The 1st and 2nd data blocks 2 and 4 are recorded being separated and between the both, unrecorded area 3 for at least one track or more is provided. Optical head 5 runs on optical disk 1 in its radius direction to reproduce data optically. Then, a data area where data have been recorded can be discriminated from the unrecorded area on the basis of the output of the optical head and the number of unrecorded areas can be counted. Therefore, random access to a desired data block can easily be attained by counting unrecorded areas 3.