PURPOSE: To convert a magneto-optical medium into a medium writable only once by correcting a medium describing table included in the control track of a medium and defining this as a new medium type for the medium.

CONSTITUTION: A data storage device comprises a data storage means 120 for reading/writing data, a data storage medium 200 placed in the data storage means 120 for storing data and a means for defining the respective storing states of a plurality of per-defined regions included in the data storage medium 200. Two locations are defined for storing state bits. In this case, the storing state bits are disposed in sector mark fields before the data parts of sectors, and by disposing these bits in the sector mark fields, continuous data writing is performed in a drive during one rotation. Thus, the medium is transformed from a rewritable medium to a medium writable only once.