A data storage medium has a first memory area that is read-only and stores first certification data that is unique to the data storage medium, a second memory area that stores data and second certification data supplied from the outside, an identity circuit for determining whether or not the first and second certification data are identical with each other, and a switch circuit for providing the data stored in the second memory area to the outside only when the identity circuit determines that the first and second certification data are identical with each other. If data is written into the data storage medium with illegal certification data, the data storage medium disables the reading of the written data from the data storage medium, thereby preventing the illegal copying of digital data.